Filed Pursuant to Rule 424(b)(5)
Registration No. 333-230723

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with the Securities and Exchange Commission and is effective. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and they are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

SUBJECT TO COMPLETION, JULY 13, 2020

PRELIMINARY PROSPECTUS SUPPLEMENT

(To prospectus dated April 12, 2019)

                 Shares of Common Stock

Pre-Funded Warrants to Purchase                  Shares of Common Stock

We are offering an aggregate of                  shares of our common stock, par value $0.0001 per share, and, to certain investors in lieu thereof, pre-funded warrants to purchase                  shares of our common stock, or the Pre-Funded Warrants, in this offering.

The purchase price of each Pre-Funded Warrant equals the public offering price per share of common stock, minus $0.0001, and the exercise price of each Pre-Funded Warrant equals $0.0001 per share. The Pre-Funded Warrants are exercisable at any time, provided that each Pre-Funded Warrant holder will be prohibited from exercising such Pre-Funded Warrants into shares of our common stock if, as a result of such exercise, the holder, together with its affiliates, would own more than 4.99% of the total number of shares of our common stock then issued and outstanding, which percentage may change at the holders’ election to any other number less than or equal to 19.99% upon 61 days’ notice to us. This prospectus supplement also relates to the offering of the shares of our common stock issuable upon exercise of such Pre-Funded Warrants.

Our common stock is currently listed on the Nasdaq Global Market under the symbol “ALT.” On July 10, 2020, the last reported sale price of our common stock on the Nasdaq Global Market was $22.35. There is no established public market for the Pre-Funded Warrants, and we do not intend to list the Pre-Funded Warrants on the Nasdaq Global Market, any other national securities exchange or any other nationally recognized trading system.

Investing in our common stock involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” on page S-10 of this prospectus supplement, and under similar headings in the documents that are incorporated by reference into this prospectus supplement and the accompanying prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or passed upon the adequacy or accuracy of this prospectus supplement and the accompanying prospectus. Any representation to the contrary is a criminal offense.

We have granted the underwriters an option for a period of 30 days to purchase up to an additional                  shares of our common stock from us at the public offering price, less underwriting discounts and commissions. If the underwriters exercise the option in full, the total underwriting discounts and commissions payable by us will be $                and the total proceeds to us, before expenses, will be $                .

Delivery of the shares of common stock and Pre-Funded Warrants is expected to be made on or about July                , 2020.

Joint Book-Running Managers

Co-Manager

Roth Capital Partners

Prospectus Supplement dated July    , 2020.

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT

PROSPECTUS

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is part of a “shelf” registration statement on Form S-3 that we filed with the SEC and is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of shares of common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part, the accompanying prospectus dated April 12, 2019, including the documents incorporated by reference therein, provides more general information. Generally, when we refer to this “prospectus,” we are referring to both parts of this document combined.

Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus, all information incorporated by reference herein and the additional information described under “Where You Can Find More Information” and “Incorporation of Documents by Reference”. These documents contain information you should consider when making your investment decision. To the extent that any statement that we make in this prospectus supplement and the accompanying prospectus is inconsistent with statements made in the accompanying prospectus or in any documents incorporated by reference, the statements made in this prospectus supplement will be deemed to modify or supersede those made in such documents incorporated by reference; however, if any statement in one of these documents is inconsistent with a statement in another document having a later date and that is incorporated by reference herein, the statement in the document having the later date modifies or supersedes the earlier statement.

You should rely only on the information contained or incorporated by reference in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and any free writing prospectus we provide you. We have not, and the underwriters have not, authorized anyone to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and any free writing prospectus we provide you is accurate only as of the date on those respective documents. Our business, financial condition, results of operations and prospects may have changed since those dates. You should read this prospectus supplement, the accompanying prospectus, including the documents incorporated by reference herein, when making your investment decision.

The distribution of this prospectus supplement and the accompanying prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States, or the U.S., who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the U.S. This prospectus supplement and the accompanying prospectus does not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless otherwise indicated, information contained in this prospectus supplement, the accompanying prospectus or the documents incorporated by reference herein concerning our industry and the markets in which we operate, including our general expectations and market position, market opportunity and market share, is based on information from our own management estimates and research, as well as from industry and general publications and research, surveys and studies conducted by third parties. Management estimates are derived from publicly available information, our knowledge of our industry and assumptions based on such information and knowledge, which we believe to be reasonable. In addition, assumptions and estimates of our and our industry’s future performance are necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described in “Risk Factors” in this prospectus supplement, and the accompanying prospectus and in our Annual Report for the year ended December 31, 2019, as filed with the SEC on March 27, 2020, as amended, supplemented or superseded from time to time by other reports we file with the SEC in the future, including our Quarterly Report for the quarter ended March 31, 2020, as filed with the SEC on May 13, 2020, which are incorporated by reference into this prospectus supplement. These and other important factors could cause our future

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performance to differ materially from our assumptions and estimates. See “Special Note Regarding Forward-Looking Statements.”

“Altimmune,” our logo and other trademarks, trade names or service marks of ours appearing in this prospectus supplement, including, HepTcell, NasoShield, NasoVAX, AdCOVID and T-COVID, are our property. The other trademarks, trade names and service marks appearing in this prospectus supplement are the property of their respective owners. We do not intend our use or display of other companies’ trademarks, trade names or service marks to imply an endorsement or sponsorship of us by such companies, or any relationship with such companies. Solely for convenience, trademarks and trade names referred to in this prospectus supplement may appear without the ^® or TM symbol.

Unless otherwise indicated or the context otherwise requires, all references in this prospectus to “we,” “us,” “our,” “Altimmune,” the “Company” and similar designations refer to Altimmune, Inc., together with its subsidiaries. General information about us can be found on our website at www.altimmune.com. The information contained on, or that can be accessed through, our website is not a part of this prospectus. We have included our website address in this prospectus solely as an inactive textual reference.

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RISK FACTORS

Investing in our common stock involves a high degree of risk. Investors should carefully consider the risks described below and under the heading “Risk Factors” in our most recent Annual Report on Form 10-K and Quarterly Reports on Form 10-Q as well as other information in this prospectus supplement and the documents incorporated by reference herein before deciding whether to invest in our securities. Such risks and uncertainties and those discussed below are not the only ones facing us. Additional risks and uncertainties not presently known to us, or that we currently see as immaterial, may also harm our business. If any of these risks occur, our business, financial condition and operating results could be harmed, the trading price of our common stock could decline and you could lose part or all of your investment.

Risks Related to This Offering

We will have broad discretion in the use of the net proceeds from this offering and may not use them effectively.

Our management will have broad discretion with respect to the use of proceeds of this offering, including for any of the purposes described in the section of this prospectus supplement titled “Use of Proceeds.” Because of the number and variability of factors that will determine our use of the net proceeds from this offering, their ultimate use may vary substantially from their currently intended use. Our failure to apply the net proceeds of this offering effectively could compromise our ability to pursue our growth strategy and we might not be able to yield a significant return, if any, on our investment of these net proceeds. You will not have the opportunity to influence our decisions on how to use the net proceeds from this offering. The failure by our management to apply these funds effectively could result in financial losses that could harm our business, cause the price of our common stock to decline and delay the development of our product candidate and any other product candidates we may develop. Pending their use, we may invest our cash, including the net proceeds from this offering, in a manner that does not produce income or that loses value. If you purchase our common stock in this offering, you will incur immediate and substantial dilution in the book value of your shares.

If you purchase securities in this offering, you will experience immediate and substantial dilution in the book value of your shares. You will experience further dilution if we issue additional equity securities in future financing transactions.

The public offering price will be substantially higher than the pro forma as adjusted net tangible book value per share of our common stock after this offering. Investors purchasing common stock or Pre-Funded Warrants in this offering will pay a price per share that substantially exceeds the pro forma as adjusted net tangible book value per share after this offering. As a result, after giving effect to (i) the exercise of 9,315,389 warrants after March 31, 2020, which provided net proceeds of $37.2 million, and (ii) the sale of 3,200,000 shares under the equity distribution agreement with JMP Securities after March 31, 2020, which provided net proceeds of $25.3 million, investors purchasing common stock in this offering will incur immediate dilution of $         per share, representing the difference between our pro forma as adjusted net tangible book value per share after giving effect to this offering and the public offering price. To the extent outstanding stock options are exercised, new stock options are issued or we issue additional shares of common stock in the future, including through the sale of equity or convertible debt securities, there will be further dilution to new investors. As a result of the dilution to investors purchasing common stock in this offering, investors may receive significantly less than the purchase price paid in this offering, if anything, in the event of our liquidation. See the section entitled “Dilution” below for a more detailed illustration of the dilution you would incur if you participate in this offering.

You may experience future dilution as a result of future equity offerings.

In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share in this offering. We may sell shares or other securities in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering.

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We do not intend to pay dividends in the foreseeable future.

We have never paid cash dividends on our common stock. We currently intend to retain our future earnings, if any, to finance the operation and growth of our business and currently do not plan to pay any cash dividends in the foreseeable future.

There is no public market for the Pre-Funded Warrants being offered in this offering.

There is no established public trading market for the Pre-Funded Warrants being offered in this offering, and we do not expect a market to develop. In addition, we do not intend to apply to list these warrants on any securities exchange or nationally recognized trading system, including the Nasdaq Global Market. Without an active market, the liquidity of these warrants will be limited.

Holders of Pre-Funded Warrants purchased in this offering will have no rights as common stockholders until such holders exercise their warrants and acquire our common stock.

Until holders of Pre-Funded Warrants acquire shares of our common stock upon exercise of such warrants, the holders will have no rights with respect to the shares of our common stock underlying such warrants. Upon exercise of the Pre-Funded Warrants, the holders will be entitled to exercise the rights of a common stockholder only as to matters for which the record date occurs after the exercise date.

Our pursuit of potential therapeutic and prophylactic treatments for COVID-19 is at an early stage and subject to many risks. We may be unable to receive approval for any of our COVID-19 product candidates a timely manner, if at all, and our COVID-19 product candidates may never be approved.

The U.S. Food and Drug Administration recently cleared our Investigational New Drug, or IND, application to proceed with a clinical trial of T-COVID, our investigational agent for the treatment of early COVID-19 and we have not yet filed an IND for AdCOVID, our intranasal vaccine candidate for COVID-19. We have not yet initiated a clinical trial for either program, and we may experience difficulties or delays in enrolling patients in clinical trials due to the impact of the global COVID-19 pandemic or other reasons. Many of the risks related to the development of these product candidates are beyond our control, including risks related to clinical development, the regulatory submission process, potential threats to our intellectual property rights and manufacturing delays or difficulties. We may be unable to produce an efficacious and/or approved product for the treatment of patients with early COVID-19 in a timely manner, if at all.

The results of preclinical studies from our COVID-19 product candidates may not be predictive of the results of clinical trials, and the results of any early-stage clinical trials we commence may not be predictive of the results of the later-stage clinical trials. There can be no assurance that any of our clinical trials for our COVID-19 product candidates, or any other of our product candidates, will ultimately be successful or support further clinical development. In addition, the interpretation of the data from our clinical trials of T-Covid or AdCOVID by FDA and other regulatory agencies may differ from our interpretation of such data and the FDA or other regulatory agencies may require that we conduct additional studies or analyses. Any of these factors could delay or prevent us from receiving regulatory approval of T-COVID or AdCOVID and there can be no assurance that either product candidate will be approved in a timely manner, if at all.

If the COVID-19 outbreak is effectively contained or the risk of coronavirus infection is diminished or eliminated before we can successfully develop and manufacture our product candidate, the commercial viability of such product candidate may be diminished or eliminated. We are also committing financial resources and personnel to the development of this product candidate which may cause delays in or otherwise negatively impact our other development programs, despite uncertainties surrounding the longevity and extent of coronavirus as a global health concern. Our business could be negatively impacted by our allocation of significant resources to a global health threat that is unpredictable and could rapidly dissipate or against which our treatment, if successfully developed, may not be effective. In addition, other parties are currently producing therapeutic and vaccine candidates for COVID-19, which may be more efficacious or may be approved prior to T-COVID or AdCOVID.

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The regulatory pathway for T-COVID and AdCOVID is continually evolving, and may result in unexpected or unforeseen challenges.

The speed at which parties are acting to create and test many therapeutics and vaccines for COVID-19 is unusual, and evolving or changing plans or priorities within the FDA, including those based on new knowledge of COVID-19 and how the disease affects the human body, may significantly affect the regulatory timeline for our product candidates. Results from ongoing clinical trials and discussions with regulatory authorities may raise new questions and require us to redesign proposed clinical trials, including revising proposed endpoints or adding new clinical trial sites or cohorts of subjects. Any such developments could delay the development timeline for our product candidates and materially increase the cost of the development for such candidates.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement and the accompanying prospectus and the documents incorporated by reference into the prospectus supplement and the accompanying prospectus contain “forward-looking statements” by us within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act, including, without limitation, statements as to expectations, beliefs and strategies regarding the future. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements. These forward-looking statements rely on a number of assumptions concerning future events and include statements relating to:

Any forward-looking statements should be considered in light of these factors. Words such as “anticipates,” “believes,” “forecasts,” “potential,” “goal,” “contemplates,” “expects,” “intends,” “plans,” “projects,” “hopes,” “seeks,” “estimates,” “strategy,” “continues,” “ongoing,” “opportunity,” “could,” “would,” “should,” “likely,” “will,” “may,” “can,” “designed to,” “future,” “foreseeable future” and similar expressions and variations, and negatives of these words, identify forward-looking statements. These forward-looking statements are based on the expectations, estimates, projections, beliefs and assumptions of our management based on information currently available to management, all of which are subject to change. These forward-looking statements are not guarantees of future performance and are subject to risks and uncertainties that could cause actual results to differ materially from the results contemplated by the forward-looking statements. Many of the important factors that will determine these results and values are beyond our ability to control or predict. You are cautioned not to put undue reliance on any forward-looking statements. Except as otherwise required by law, we do not assume any obligation to update any forward-looking statements.

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In evaluating an investment in shares of our common stock, you should carefully consider the discussion of risks and uncertainties described under the heading “Risk Factors” contained in this prospectus supplement, and under similar headings in other documents, including in, and, and in other filings with the SEC, which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future, that are incorporated by reference in this prospectus supplement. You should carefully read this prospectus supplement together with the information incorporated by reference in this prospectus supplement as described under the headings “Where You Can Find More Information” and “Incorporation of Documents by Reference” completely and with the understanding that our actual future results may be materially different from what we expect.

All subsequent written and oral forward-looking statements attributable to us or persons acting on our behalf are expressly qualified in their entirety by our cautionary statements. The forward-looking statements included or incorporated by reference herein are made only as of the date of this prospectus supplement (or as of the date of any such document incorporated by reference). We do not intend, and undertake no obligation, to update these forward-looking statements, except as required by law.

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USE OF PROCEEDS

Based on the public offering price of $         per share of common stock, we estimate that the net proceeds to us from the sale of the shares of common stock and Pre-Funded Warrants that we are offering will be approximately $         million, or approximately $         million, if the option to purchase additional shares is exercised in full by the underwriters, in each case, after deducting underwriting discounts, commissions and estimated offering expenses payable by us.

We currently intend to use the net proceeds from this offering for working capital and general corporate purposes, which may include, but are not limited to, scale up of manufacturing and advanced clinical trials of AdCOVID; the continued development of ALT-801, including manufacturing and clinical trials; and for capital expenditures and working capital.

The precise amount and timing of the application of these proceeds will depend upon a number of factors, such as the timing and progress of our research and development efforts, our funding requirements and the availability and costs of other funds. As of the date of this prospectus supplement, we cannot specify with certainty all of the particular uses for the net proceeds to us from this offering. Accordingly, our management will have broad discretion in the timing and application of these proceeds. Pending application of the net proceeds as described above, we intend to temporarily invest the proceeds in short-term, interest-bearing instruments.

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DILUTION

If you purchase our common stock in this offering, your interest will be diluted to the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock after this offering.

Our net tangible book value at March 31, 2020 was approximately $29.2 million, or $1.90 per share. We calculate net tangible book value per share by dividing our net tangible assets (tangible assets less total liabilities) by the number of shares of our common stock issued and outstanding as of March 31, 2020. Dilution in net tangible book value per share represents the difference between the amount per share paid by purchasers of common stock in this offering and the net tangible book value per common stock immediately after this offering.

Our pro forma net tangible book value as of March 31, 2020 was $91.7 million, or $3.29 per share. Pro forma net tangible book value represents the amount of our total tangible assets less our total liabilities, after giving effect to (i) the exercise of 9,315,389 warrants subsequent to March 31, 2020, which provided net proceeds of $37.2 million and (ii) the sale of 3,200,000 shares under the equity distribution agreement with JMP Securities subsequent to March 31, 2020, which provided net proceeds of $25.3 million.

After giving effect to the sale by us of                  shares of our common stock in this offering at a public offering price of $         per share of common stock, and after deducting underwriting discounts and commissions and estimated aggregate offering expenses payable by us, our pro forma as adjusted net tangible book value as of March 31, 2020 would have been approximately $         million, or $         per share of common stock. This represents an immediate increase in the net tangible book value of $         per share to our existing stockholders and an immediate dilution in net tangible book value of $         per share to new investors. The following table illustrates this per share dilution:

If the underwriters exercise in full their option to purchase up to                  additional shares of common stock at the public offering price of $         per share, the pro forma as adjusted net tangible book value after this offering would be approximately $         million, or $         per share, representing an immediate increase in net tangible book value of $         per share to existing stockholders and immediate dilution in net tangible book value of $         per share to investors purchasing our common stock in this offering.

The discussion and table above are based on 15,359,502 shares outstanding as of March 31, 2020 which excludes:

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Except as otherwise indicated, all information in this prospectus supplement assumes no exercise of stock options or warrants after March 31, 2020.

If any shares are issued upon exercise of the outstanding warrants to purchase 1,056,175 shares of our common stock as of July 10, 2020, you will experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic considerations, even if we believe we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

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DESCRIPTION OF PRE-FUNDED WARRANTS

Pre-Funded Warrants

General

The term “pre-funded” refers to the fact that the purchase price of the Pre-Funded Warrants in this offering includes almost the entire exercise price that will be paid under the Pre-Funded Warrants, except for a nominal remaining exercise price of $0.0001. The purpose of the Pre-Funded Warrants is to enable investors that may have restrictions on their ability to beneficially own more than 4.99% of our outstanding common stock following the consummation of this offering the opportunity to invest capital into the Company without triggering their ownership restrictions, by receiving Pre-Funded Warrants in lieu of shares of our common stock which would result in such ownership of more than 4.99% and receiving the ability to exercise their option to purchase the shares underlying the Pre-Funded Warrants at a nominal price at a later date.

The following is a brief summary of certain terms and conditions of the Pre-Funded Warrants being offered by this prospectus supplement. The following description is subject in all respects to the provisions contained in the Pre-Funded Warrants.

Terms of the Pre-Funded Warrants

Form

The Pre-Funded Warrants will be issued as individual warrant agreements to the investors. The form of Pre-Funded Warrant will be filed as an exhibit to our Current Report on Form 8-K that we expect to file with the SEC in connection with this offering.

Exercise price

Pre-Funded Warrants will have an exercise price of $0.0001 per share. The exercise price is subject to appropriate adjustment in the event of certain stock dividends and distributions, stock splits, stock combinations, reclassifications or similar events affecting our common stock and also upon any distributions of assets, including cash, stock or other property to our stockholders.

Exercisability

The Pre-Funded Warrants are exercisable at any time after their original issuance. The Pre-Funded Warrants will be exercisable, at the option of each holder, in whole or in part by delivering to us a duly executed exercise notice and by payment in full of the exercise price in immediately available funds for the number of shares of common stock purchased upon such exercise. As an alternative to payment in immediately available funds, the holder may elect to exercise the Pre-Funded Warrant through a cashless exercise, in which the holder would receive upon such exercise the net number of shares of common stock determined according to the formula set forth in the Pre-Funded Warrant. No fractional shares of common stock will be issued in connection with the exercise of a Pre-Funded Warrant.

Exercise limitations

The Pre-Funded Warrants may not be exercised by the holder to the extent that the holder, together with its affiliates, would beneficially own, after such exercise more than 4.99% of the shares of our common stock then outstanding (including for such purpose the shares of our common stock issuable upon such exercise). However, any holder may increase or decrease such beneficial ownership limitation upon notice to us, provided that such limitation cannot exceed 19.99%, and provided that any increase in the beneficial ownership limitation shall not be effective until 61 days after such notice is delivered.

Term

The Pre-Funded Warrants do not expire.

Transferability

Subject to applicable laws, the Pre-Funded Warrants may be offered for sale, sold, transferred or assigned without our consent.

Exchange listing

There is no established trading market for the Pre-Funded Warrants and we do not expect a market to develop. In addition, we do not intend to apply for the listing of the Pre-Funded Warrants on any national securities exchange or other trading market. Without an active trading market, the liquidity of the Pre-Funded Warrants will be limited.

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Fundamental transactions

In the event of a fundamental transaction, as described in the Pre-Funded Warrants and generally including any reorganization, recapitalization or reclassification of our common stock, the sale, transfer or other disposition of all or substantially all of our properties or assets, our consolidation or merger with or into another person, the acquisition of more than 50% of our outstanding common stock, or any person or group becoming the beneficial owner of 50% of the voting power represented by our outstanding common stock, upon consummation of such a fundamental transaction, the holders of the Pre-Funded Warrants will be entitled to receive upon exercise of the Pre-Funded Warrants the kind and amount of securities, cash or other property that the holders would have received had they exercised the Pre-funded Warrants immediately prior to such fundamental transaction without regard to any limitations on exercise contained in the Pre-Funded Warrants.

No rights as a stockholder

Except by virtue of such holder’s ownership of shares of our common stock, the holder of a Pre-Funded Warrant does not have the rights or privileges of a holder of our common stock, including any voting rights, until the holder exercises the Pre-Funded Warrant.

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MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS FOR NON-U.S. HOLDERS

The following is a summary of the material U.S. federal income tax considerations applicable to non-U.S. holders (defined below) with respect to their ownership and disposition of our common stock or Pre-Funded Warrants, but does not purport to be a complete analysis of all the potential tax considerations relating thereto. This summary is based upon the provisions of the Internal Revenue Code of 1986, as amended, or the Code, Treasury regulations promulgated thereunder, administrative rulings and judicial decisions, all as of the date hereof. These authorities may be changed or subject to differing interpretations, possibly with retroactive effect, so as to result in U.S. federal income tax consequences different from those set forth below. We have not sought any ruling from the Internal Revenue Service (the IRS) with respect to the statements made and the conclusions reached in the following summary, and there can be no assurance that the IRS will agree with such statements and conclusions.

This summary also does not address the tax considerations arising under the laws of any U.S. state or local or any non-U.S. jurisdiction, U.S. federal estate or gift tax laws, the Medicare tax on net investment income or any alternative minimum tax consequences. In addition, this discussion does not address tax considerations applicable to an investor’s particular circumstances or to investors that may be subject to special tax rules, including, without limitation:

In addition, if a partnership, including any entity or arrangement classified as a partnership for U.S. federal income tax purposes, holds our common stock or Pre-Funded Warrants, the tax treatment of a partner generally will depend on the status of the partner and upon the activities of the partnership. Accordingly, partnerships that hold our common stock or Pre-Funded Warrants, and partners in such partnerships, should consult their tax advisors.

You are urged to consult your tax advisor with respect to the application of the U.S. federal income tax laws to your particular situation, as well as any tax consequences of the purchase, ownership and disposition of our common stock or Pre-Funded Warrants arising under the U.S. federal estate or gift tax rules or under the laws of any U.S. state or local or any non-U.S. or other taxing jurisdiction or under any applicable tax treaty.

For purposes of this discussion, you are a non-U.S. holder if you are a beneficial owner (other than a partnership or an entity or arrangement treated as a partnership for U.S. federal income tax purposes) of our common stock or Pre-Funded Warrants that is for U.S. federal income tax purposes

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Pre-Funded Warrants

Although the law in this area is not completely settled, the Pre-Funded Warrants are generally expected to be treated as outstanding shares of our common stock for U.S. federal income tax purposes and a holder of Pre-Funded Warrants should generally be taxed in the same manner as a holder of such common stock. Accordingly, no gain or loss should be recognized upon the exercise of a Pre-Funded Warrant, and, upon exercise, the holding period of a Pre-Funded Warrant should carry over to the shares of common stock received. In addition, the tax basis of the Pre-Funded Warrant should carry over to the shares of common stock received upon exercise, increased by the exercise price of $0.0001 per share. Our characterization is not binding on the IRS, and the IRS could treat our Pre-Funded Warrants as warrants to acquired our common stock. In that case, the tax treatment, including the amount and character of any gain with respect to an investment in our Pre-Funded Warrants could be materially different than the discussion set forth below. If you are a non-U.S. holder that is contemplating the acquisition of Pre-Funded Warrants, you should discuss with your personal tax advisor the consequences of the purchase, ownership and disposition of the Pre-Funded Warrants, as well as the exercise of the Pre-Funded Warrants into our common stock. The balance of this discussion generally assumes that the characterization described above is respected for U.S. federal income tax purposes.

Distributions on our Common Stock

As discussed under “Dividend Policy,” above, we do not anticipate paying any dividends on our capital stock in the foreseeable future. If we were to make distributions on our common stock, those payments will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. To the extent those distributions exceed both our current and our accumulated earnings and profits, they will constitute a return of capital and will first reduce your basis in our common stock, but not below zero, and then will be treated as gain from the sale of stock, subject to the tax treatment described in the discussion below regarding taxable dispositions of our common stock. Any such distributions would also be subject to the discussions below regarding backup withholding and FATCA.

Subject to the discussion below regarding a dividend received by you that is effectively connected with the conduct of a U.S. trade or business, a dividend paid to you generally will be subject to U.S. withholding tax either at a rate of 30% of the gross amount of the dividend or such lower rate as may be specified by an applicable income tax treaty. In order to receive a reduced treaty rate, you must provide us with an IRS Form W-8BEN (generally including a U.S. taxpayer identification number), IRS Form W-8BEN-E or another appropriate version of IRS Form W-8 (or a successor form), in each case, certifying qualification for the reduced rate.

Dividends received by you that are effectively connected with the conduct of a U.S. trade or business (and, if required by an applicable income tax treaty, are attributable to a permanent establishment maintained by you in the United States) generally are exempt from such withholding tax. In order to obtain this exemption, you must provide us with an IRS Form W-8ECI or successor form or other applicable IRS Form W-8 properly certifying such exemption. Such effectively connected dividends, although not subject to withholding tax, are taxed at the same graduated rates applicable to U.S. persons, net of certain deductions and credits, subject to an applicable income tax treaty providing otherwise. In addition, if you are a corporate non-U.S. holder, you may also be subject to a branch profits tax at a rate of 30% or such lower rate as may be specified by an applicable income tax treaty on such effectively connected dividends.

If you are eligible for a reduced rate of withholding tax pursuant to a tax treaty, you may be able to obtain a refund of any excess amounts currently withheld if you file an appropriate claim for refund with the IRS.

The Pre-Funded Warrants are not entitled to any distributions until such warrant is exercised. Upon the exercise of a Pre-Funded Warrant, a holder shall be entitled to receive distributions declared on our common stock prior to the exercise of such Pre-Funded Warrant. It is possible that such entitlement to distributions may cause the declaration

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of a distribution on our common stock to be currently taxable to holders of Pre-Funded Warrants under the principles governing Section 305 of the Code. Each holder should consult his, her or its own tax advisor regarding the potential taxation of distributions declared on our common stock.

Constructive Dividends on Pre-Funded Warrants.

Under Section 305 of the Code, an adjustment to (or failure to adjust) the number of shares that will be issued on the exercise of the Pre-Funded Warrants, or an adjustment to (or failure to adjust) the exercise price of the Pre-Funded Warrants, may be treated as a constructive distribution to a non-U.S. holder of the Pre-Funded Warrants if, and to the extent that, such adjustment (or failure to adjust) has the effect of increasing such U.S. holder’s proportionate interest in our assets or earnings and profits as determined under U.S. federal income tax principles, depending on the circumstances of such adjustment (for example, if such adjustment is to compensate for a distribution of cash or other property to our shareholders) The Pre-Funded Warrants are not entitled to any distributions until such Pre-Funded Warrant is exercised. Upon the exercise of a Pre-Funded Warrant, a holder shall be entitled to receive distributions declared on our common stock prior to the exercise of such Pre-Funded Warrant. It is possible that such entitlement to distributions may cause the declaration of a distribution on our common stock to be currently taxable to holders of Pre-Funded Warrants under the principles governing Section 305 of the Code. Non-U.S. holders should consult their tax advisors as to the general treatment of constructive distributions under their particular circumstances. Because a constructive dividend deemed received by a U.S. holder would not give rise to any cash from which any applicable withholding could be satisfied, such withholding may be set off against payments on the Pre-Funded Warrants or common stock.

Gain on Sale or Other Taxable Disposition of Common Stock and/or Pre-Funded Warrants

Subject to the discussion below regarding backup withholding and FATCA, you generally will not be required to pay U.S. federal income tax on any gain realized upon the sale or other taxable disposition of our common stock and/or Pre-Funded Warrants unless:

You should consult your own tax advisor regarding any applicable tax treaties that may provide for different rules.

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Backup Withholding and Information Reporting

Generally, we must report annually to the IRS the amount of dividends paid to you, your name and address, and the amount of tax withheld, if any. A similar report will generally be sent to you. Pursuant to applicable income tax treaties or other agreements, the IRS may make these reports available to tax authorities in your country of residence.

Payments of dividends or of proceeds on the disposition of our common stock and/or Pre-Funded Warrants made to you may be subject to additional information reporting and backup withholding at the then applicable rate unless you establish an exemption, for example by properly certifying your non-U.S. status on an IRS Form W-8BEN, IRS Form W-8BEN-E or another appropriate version of IRS Form W-8 (or a successor form). Notwithstanding the foregoing, backup withholding and information reporting may apply if either we or our paying agent has actual knowledge, or reason to know, that you are a U.S. person.

Information reporting and backup withholding will generally apply to the proceeds of a disposition of our common stock and/or Pre-Funded Warrants effected by you by or through the U.S. office of any broker, U.S. or foreign, unless you certify your status as a non-U.S. holder and satisfy certain other requirements, or otherwise establish an exemption. Generally, information reporting and backup withholding will not apply to a payment of disposition proceeds to a non-U.S. holder where the transaction is effected outside the United States through a non-U.S. office of a broker. However, for information reporting purposes, dispositions effected through a non-U.S. office of a broker with substantial U.S. ownership or operations generally will be treated in a manner similar to dispositions effected through a U.S. office of a broker.

Backup withholding is not an additional tax; rather, the U.S. federal income tax liability of persons subject to backup withholding will be reduced by the amount of tax withheld. If withholding results in an overpayment of taxes, a refund or credit may generally be obtained from the IRS, provided that the required information is furnished to the IRS in a timely manner.

Foreign Account Tax Compliance Act (FATCA)

Provisions commonly referred to as “FATCA” may impose withholding tax on certain types of payments made to “foreign financial institutions” and certain other non-U.S. entities. The legislation imposes a 30% withholding tax on dividends on our common stock paid to a foreign financial institution or to certain non-financial foreign entities, unless (i) the foreign financial institution undertakes certain diligence, reporting and withholding obligations, (ii) the non-financial foreign entity either certifies it does not have any substantial U.S. owners or furnishes identifying information regarding each substantial U.S. owner and such entity meets certain other specified requirements, or (iii) the foreign financial institution or non-financial foreign entity otherwise qualifies for an exemption from these rules. Such withholding may also apply to gross proceeds from the sale or other disposition of our common stock and/or Pre-Funded Warrants, although under proposed U.S. Treasury Regulations, no withholding would apply to such gross proceeds. The preamble to the proposed regulations specifies that taxpayers (including withholding agents) are permitted to rely on the proposed regulations pending finalization. If withholding under FATCA is required on any payment related to our common stock and/or Pre-Funded Warrants, under certain circumstances, a non-U.S. holder may be eligible for refunds or credits of this withholding tax. An intergovernmental agreement between the United States and an applicable foreign country may modify the requirements described in this paragraph. Non-U.S. holders should consult their tax advisors regarding the possible implications of FATCA on their investment in our common stock and/or Pre-Funded Warrants and the entities through which they hold our common stock.

The preceding discussion of U.S. federal income tax considerations is for information only. It is not tax advice. Each prospective investor should consult its own tax advisor regarding the particular U.S. federal, state and local and non-U.S. tax consequences of purchasing, holding and disposing of our common stock and/or Pre-Funded Warrants, including the consequences of any proposed change in applicable laws.

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UNDERWRITING

Subject to the terms and conditions set forth in the underwriting agreement among us and Jefferies LLC, Evercore Group L.L.C. and Piper Sandler & Co., as the representatives of the underwriters named below and the joint book-running managers of this offering, we have agreed to sell to the underwriters, and each of the underwriters has agreed, severally and not jointly, to purchase from us, the respective number of shares of common stock and Pre-Funded Warrants shown opposite its name below:

The underwriting agreement provides that the obligations of the several underwriters are subject to certain conditions precedent such as the receipt by the underwriters of officers’ certificates and legal opinions and approval of certain legal matters by their counsel. The underwriting agreement provides that the underwriters will purchase all of the shares of common stock and Pre-Funded Warrants if any of them are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the nondefaulting underwriters may be increased or the underwriting agreement may be terminated. We have agreed to indemnify the underwriters and certain of their controlling persons against certain liabilities, including liabilities under the Securities Act, and to contribute to payments that the underwriters may be required to make in respect of those liabilities.

The underwriters have advised us that, following the completion of this offering, they currently intend to make a market in the common stock as permitted by applicable laws and regulations. However, the underwriters are not obligated to do so, and the underwriters may discontinue any market-making activities at any time without notice in their sole discretion. Accordingly, no assurance can be given as to the liquidity of the trading market for the common stock, that you will be able to sell any of the common stock held by you at a particular time or that the prices that you receive when you sell will be favorable.

The underwriters are offering the shares of common stock and Pre-Funded Warrants subject to their acceptance of the shares of common stock and Pre-Funded Warrants from us and subject to prior sale. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Commission and Expenses

The underwriters have advised us that they propose to offer the shares of common stock to the public at the public offering price set forth on the cover page of this prospectus supplement, and the Pre-Funded Warrants to certain dealers, which may include the underwriters, at that price less a concession not in excess of $              per share of common stock. After the offering, the public offering price, concession and reallowance to dealers may be reduced by the representatives. No such reduction will change the amount of proceeds to be received by us as set forth on the cover page of this prospectus supplement.

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The following table shows the public offering price, the underwriting discounts and commissions that we are to pay the underwriters and the proceeds, before expenses, to us in connection with this offering. Such amounts are shown assuming both no exercise and full exercise of the underwriters’ option to purchase additional shares.

We estimate expenses payable by us in connection with this offering, other than the underwriting discounts and commissions referred to above, will be approximately $            . We have also agreed to reimburse the underwriters for up to $15,000 expenses incurred by them in connection with the offering.

Listing

Our common stock is listed on the Nasdaq Global Market under the symbol “ALT.” There is no established public market for the Pre-Funded Warrants, and we do not intend to list the Pre-Funded Warrants on the Nasdaq Global Market, any other national securities exchange or any other nationally recognized trading system.

Option to Purchase Additional Shares

We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase, from time to time, in whole or in part, up to an aggregate of                shares from us at the public offering price set forth on the cover page of this prospectus supplement, less underwriting discounts and commissions. The number of shares subject to the underwriters’ option will equal 15% of the total number of shares of common stock we are offering plus the shares underlying the Pre-Funded Warrants. If the underwriters exercise this option, each underwriter will be obligated, subject to specified conditions, to purchase a number of additional shares proportionate to that underwriter’s initial purchase commitment as indicated in the table above.

No Sales of Similar Securities

We, our officers and our directors have agreed, subject to specified exceptions, not to directly or indirectly:

These restrictions terminate after the close of trading of the common stock on and including the 90th day after the date of this prospectus supplement.

Jefferies LLC, Evercore Group L.L.C. and Piper Sandler & Co. may, in their sole discretion and at any time or from time to time before the termination of the 90-day period release all or any portion of the securities subject to lock-up agreements.

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Stabilization

The underwriters have advised us that, pursuant to Regulation M under the Exchange Act, they and certain persons participating in the offering, may engage in short sale transactions, stabilizing transactions, syndicate covering transactions or the imposition of penalty bids in connection with this offering. These activities may have the effect of stabilizing or maintaining the market price of the common stock at a level above that which might otherwise prevail in the open market. Establishing short sales positions may involve either “covered” short sales or “naked” short sales.

“Covered” short sales are sales made in an amount not greater than the underwriters’ option to purchase additional shares of our common stock in this offering. The underwriters may close out any covered short position by either exercising their option to purchase additional shares of our common stock or purchasing shares of our common stock in the open market. In determining the source of shares to close out the covered short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase shares through the option to purchase additional shares.

“Naked” short sales are sales in excess of the option to purchase additional shares of our common stock. The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the shares of our common stock in the open market after pricing that could adversely affect investors who purchase in this offering.

A stabilizing bid is a bid for the purchase of shares of common stock on behalf of the underwriters for the purpose of fixing or maintaining the price of the common stock. A syndicate covering transaction is the bid for or the purchase of shares of common stock on behalf of the underwriters to reduce a short position incurred by the underwriters in connection with the offering. Similar to other purchase transactions, the underwriter’s purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. A penalty bid is an arrangement permitting the underwriters to reclaim the selling concession otherwise accruing to a syndicate member in connection with the offering if the common stock originally sold by such syndicate member are purchased in a syndicate covering transaction and therefore have not been effectively placed by such syndicate member.

Neither we nor any of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. The underwriters are not obligated to engage in these activities and, if commenced, any of the activities may be discontinued at any time.

The underwriters may also engage in passive market making transactions in our common stock on the Nasdaq Global Market in accordance with Rule 103 of Regulation M during a period before the commencement of offers or sales of shares of our common stock in this offering and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, that bid must then be lowered when specified purchase limits are exceeded.

Electronic Distribution

This prospectus supplement and the accompanying prospectus in electronic format may be made available by e-mail or on the web sites or through online services maintained by one or more of the underwriters or their affiliates. In those cases, prospective investors may view offering terms online and may be allowed to place orders online. The underwriters may agree with us to allocate a specific number of shares of common stock for sale to online brokerage account holders. Any such allocation for online distributions will be made by the underwriters on the same basis as other allocations. Other than this prospectus supplement and the accompanying prospectus in electronic format, the information on the underwriters’ web sites and any information contained in any other web site maintained by any of the underwriters is not part of this prospectus supplement or the accompanying prospectus, has not been approved and/or endorsed by us or the underwriters and should not be relied upon by investors.

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Other Activities and Relationships

The underwriters and certain of their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. The underwriters and certain of their respective affiliates have, from time to time, performed, and may in the future perform, various commercial and investment banking and financial advisory services for us and our affiliates, for which they received or will receive customary fees and expenses.

In the ordinary course of their various business activities, the underwriters and certain of their respective affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers, and such investment and securities activities may involve securities and/or instruments issued by us and our affiliates. If the underwriters or their respective affiliates have a lending relationship with us, they routinely hedge their credit exposure to us consistent with their customary risk management policies. The underwriters and their respective affiliates may hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities or the securities of our affiliates, including potentially the common stock offered hereby. Any such short positions could adversely affect future trading prices of the common stock offered hereby. The underwriters and certain of their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Disclaimers About Non-U.S. Jurisdictions

Canada

Resale Restrictions

The distribution of securities in Canada is being made only in the provinces of Ontario, Quebec, Alberta and British Columbia on a private placement basis exempt from the requirement that we prepare and file a prospectus with the securities regulatory authorities in each province where trades of these securities are made. Any resale of the securities in Canada must be made under applicable securities laws which may vary depending on the relevant jurisdiction, and which may require resales to be made under available statutory exemptions or under a discretionary exemption granted by the applicable Canadian securities regulatory authority. Purchasers are advised to seek legal advice prior to any resale of the securities.

Representations of Canadian Purchasers

By purchasing securities in Canada and accepting delivery of a purchase confirmation, a purchaser is representing to us and the dealer from whom the purchase confirmation is received that:

Conflicts of Interest

Canadian purchasers are hereby notified that the underwriters are relying on the exemption set out in section 3A.3 or 3A.4, if applicable, of National Instrument 33-105—Underwriting Conflicts from having to provide certain conflict of interest disclosure in this document.

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Statutory Rights of Action

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if the prospectus (including any amendment thereto) such as this document contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser of these securities in Canada should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Enforcement of Legal Rights

All of our directors and officers as well as the experts named herein may be located outside of Canada and, as a result, it may not be possible for Canadian purchasers to effect service of process within Canada upon us or those persons. All or a substantial portion of our assets and the assets of those persons may be located outside of Canada and, as a result, it may not be possible to satisfy a judgment against us or those persons in Canada or to enforce a judgment obtained in Canadian courts against us or those persons outside of Canada.

Taxation and Eligibility for Investment

Canadian purchasers of securities should consult their own legal and tax advisors with respect to the tax consequences of an investment in the securities in their particular circumstances and about the eligibility of the securities for investment by the purchaser under relevant Canadian legislation.

Australia

Neither this prospectus supplement nor the accompanying prospectus is a disclosure document for the purposes of Australia’s Corporations Act 2001 (Cth) of Australia, or Corporations Act, or has been lodged with the Australian Securities & Investments Commission, and each is only directed to the categories of exempt persons set out below. Accordingly, if you receive this prospectus supplement and the accompanying prospectus in Australia, you confirm and warrant that you are either:

To the extent that you are unable to confirm or warrant that you are an exempt sophisticated investor, associated person or professional investor under the Corporations Act, any offer made to you under this prospectus supplement and the accompanying prospectus is void and incapable of acceptance.

You warrant and agree that you will not offer any of the securities issued to you pursuant to this prospectus supplement and the accompanying prospectus for resale in Australia within 12 months of those securities being issued unless any such resale offer is exempt from the requirement to issue a disclosure document under section 708 of the Corporations Act.

European Economic Area

In relation to each Member State of the European Economic Area (each a “Member State”), no securities have been offered or will be offered pursuant to this offering to the public in that Member State prior to the publication of a prospectus in relation to the securities which has been approved by the competent authority in that Member State or, where appropriate, approved in another Member State and notified to the competent authority in that Member State, all in accordance with the Prospectus Regulation, except that offers of securities may be made to the public in that Member State at any time under the following exemptions under the Prospectus Regulation:

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provided that no such offer of securities shall require us or any underwriter to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the Prospectus Regulation.

For the purposes of this provision, the expression an “offer to the public” in relation to any offer of securities in any Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any securities to be offered so as to enable an investor to decide to purchase or subscribe for the securities and the expression “Prospectus Regulation” means Regulation (EU) 2017/1129.

Hong Kong

No securities have been offered or sold, and no securities may be offered or sold, in Hong Kong, by means of any document, other than to persons whose ordinary business is to buy or sell shares or debentures, whether as principal or agent; or to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong, or SFO, and any rules made under that Ordinance; or in other circumstances which do not result in the document being a “prospectus” as defined in the Companies Ordinance (Cap. 32) of Hong Kong, or CO, or which do not constitute an offer or invitation to the public for the purpose of the CO or the SFO. No document, invitation or advertisement relating to the securities has been issued or may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted under the securities laws of Hong Kong) other than with respect to securities which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the SFO and any rules made under that Ordinance.

Neither this prospectus supplement nor the accompanying prospectus have been registered with the Registrar of Companies in Hong Kong. Accordingly, this prospectus supplement and the accompanying prospectus may not be issued, circulated or distributed in Hong Kong, and the securities may not be offered for subscription to members of the public in Hong Kong. Each person acquiring the securities will be required, and is deemed by the acquisition of the securities, to confirm that he is aware of the restriction on offers of the securities described in this prospectus supplement and the accompanying prospectus and the relevant offering documents and that he is not acquiring, and has not been offered any securities in circumstances that contravene any such restrictions.

Israel

This prospectus supplement and the accompanying prospectus do not constitute a prospectus under the Israeli Securities Law, 5728-1968, or the Securities Law, and has not been filed with or approved by the Israel Securities Authority. In Israel, this prospectus supplement and the accompanying prospectus are being distributed only to, and are directed only at, and any offer of the securities is directed only at, (i) a limited number of persons in accordance with the Israeli Securities Law and (ii) investors listed in the first addendum, or the Addendum, to the Israeli Securities Law, consisting primarily of joint investment in trust funds, provident funds, insurance companies, banks, portfolio managers, investment advisors, members of the Tel Aviv Stock Exchange, underwriters, venture capital funds, entities with equity in excess of NIS 50 million and “qualified individuals,” each as defined in the Addendum (as it may be amended from time to time), collectively referred to as qualified investors (in each case, purchasing for their own account or, where permitted under the Addendum, for the accounts of their clients who are investors listed in the Addendum). Qualified investors are required to submit written confirmation that they fall within the scope of the Addendum, are aware of the meaning of same and agree to it.

France

Neither this prospectus supplement, the accompanying prospectus nor any other offering material relating to the securities described in this prospectus supplement or the accompanying prospectus has been submitted to the clearance procedures of the Autorité des Marchés Financiers or of the competent authority of another member state of the European Economic Area and notified to the Autorité des Marchés Financiers. The securities have not been offered or sold and will not be offered or sold, directly or indirectly, to the public in France. Neither this prospectus supplement nor any other offering material relating to the securities has been or will be:

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Such offers, sales and distributions will be made in France only:

The securities may be resold directly or indirectly, only in compliance with articles L.411-1, L.411-2, L.412-1 and L.621-8 through L.621-8-3 of the French Code monétaire et financier.

Japan

The offering has not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948 of Japan, as amended), or FIEL, and the underwriters will not offer or sell any securities, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan (which term as used herein means, unless otherwise provided herein, any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or resale, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the FIEL and any other applicable laws, regulations and ministerial guidelines of Japan.

Singapore

Neither this prospectus supplement nor the accompanying prospectus has been and neither will be lodged or registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement and the accompanying prospectus and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the securities may not be circulated or distributed, nor may the securities be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore, or the SFA, (ii) to a relevant person pursuant to Section 275(1), or any person pursuant to Section 275(1A), and in accordance with the conditions specified in Section 275, of the SFA, or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

Where the securities are subscribed or purchased under Section 275 of the SFA by a relevant person, which is:

securities (as defined in Section 239(1) of the SFA) of that corporation or the beneficiaries’ rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the securities pursuant to an offer made under Section 275 of the SFA except:

Solely for the purposes of its obligations pursuant to Section 309B of the SFA, we have determined, and hereby notify all relevant persons (as defined in the CMP Regulations 2018), that the securities are “prescribed capital

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markets products” (as defined in the CMP Regulations 2018) and Excluded Investment Products (as defined in MAS Notice SFA 04-N12: Notice on the Sale of Investment Products and MAS Notice FAA-N16: Notice on Recommendations on Investment Products).

Switzerland

The securities may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange, or SIX, or on any other stock exchange or regulated trading facility in Switzerland. This prospectus supplement and the accompanying prospectus have been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this prospectus supplement, the accompanying prospectus nor any other offering or marketing material relating to the securities or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this prospectus supplement, the accompanying prospectus nor any other offering or marketing material relating to this offering, us or the securities have been or will be filed with or approved by any Swiss regulatory authority. In particular, this prospectus supplement and the accompanying prospectus will not be filed with, and the offer of securities will not be supervised by, the Swiss Financial Market Supervisory Authority FINMA, and the offer of securities has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes. The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of securities.

United Kingdom

This prospectus supplement and the accompanying prospectus are only being distributed to, and are only directed at, persons in the United Kingdom that are qualified investors within the meaning of Article 2(1)(e) of the Prospectus Regulation that are also (i) investment professionals falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, or the Order, and/or (ii) who are high net worth companies (or persons to whom it may otherwise be lawfully communicated) falling within Article 49(2)(a) to (d) of the Order (all such persons together being referred to as “relevant persons”) or otherwise in circumstances which have not resulted and will not result in an offer to the public of the securities in the United Kingdom within the meaning of the Financial Services and Markets Act 2000.

Any person in the United Kingdom that is not a relevant person should not act or rely on the information included in this document or use it as basis for taking any action. In the United Kingdom, any investment or investment activity that this document relates to may be made or taken exclusively by relevant persons.

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LEGAL MATTERS

The validity of the issuance of the securities offered by this prospectus supplement will be passed upon for us by Goodwin Procter LLP. Cooley LLP is acting as counsel for the underwriters in connection with certain legal matters related to this offering.

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EXPERTS

The consolidated financial statements of Altimmune, Inc. appearing in its Annual Report (Form 10-K) for the year ended December 31, 2019, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

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WHERE YOU CAN FIND MORE INFORMATION

We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the SEC. We have filed with the SEC a registration statement on Form S-3 under the Securities Act with respect to the securities being offered under this prospectus supplement and the accompanying prospectus. This prospectus supplement and the accompanying prospectus do not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities being offered under this prospectus supplement and the accompanying prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. The SEC maintains an Internet site that contains reports, proxy and information statements, and other information regarding issuers that file electronically with the SEC, including Altimmune, Inc. The SEC’s Internet site can be found at http://www.sec.gov.

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INCORPORATION OF DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” information into this prospectus supplement, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The documents incorporated by reference into this prospectus supplement contain important information that you should read about us.

The following documents are incorporated by reference into this prospectus supplement:

We also incorporate by reference any future filings (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items unless such Form 8-K expressly provides to the contrary) made with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, including those made after the date of the initial filing of the registration statement of which this prospectus supplement is a part and prior to effectiveness of such registration statement, until we file a post-effective amendment that indicates the termination of the offering of the securities made by this prospectus supplement and will become a part of this prospectus supplement from the respective dates that such documents are filed with the SEC. Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes hereof or of the related prospectus supplement to the extent that a statement contained herein or in any other subsequently filed document which is also incorporated or deemed to be incorporated herein modifies or supersedes such statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus supplement.

Documents incorporated by reference are available from us, without charge. You may obtain documents incorporated by reference in this prospectus supplement by requesting them in writing or by telephone at the following address:

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, Maryland

Attention: Chief Financial Officer

(240) 654-1450

S-35

PROSPECTUS

ALTIMMUNE, INC.

$250,000,000

COMMON STOCK

PREFERRED STOCK

DEPOSITARY SHARES

DEBT SECURITIES

WARRANTS

STOCK PURCHASE CONTRACTS

STOCK PURCHASE UNITS

We may offer and sell from time to time:

We may offer and sell these securities to or through one or more underwriters, dealers and agents, or directly to purchasers, on a continuous or delayed basis. This prospectus describes some of the general terms that may apply to these securities and the general manner in which they may be offered. The specific terms of any securities to be offered, and the specific manner in which they may be offered, will be described in a supplement to this prospectus. You should read this prospectus and each applicable prospectus supplement carefully before you invest. This prospectus may not be used to sell securities unless accompanied by a prospectus supplement.

Our common stock is listed on the Nasdaq Global Market under the symbol “ALT.” We have not yet determined whether any of the other securities that may be offered by this prospectus and a prospectus supplement will be listed on any exchange, inter-dealer quotation system or over-the-counter market. Our principal executive offices are located at 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878, and its telephone number is (240) 654-1450.

Investing in our securities involves risks. You should carefully read and consider the risk factors included in our periodic reports and other information that we file with the Securities and Exchange Commission and that are incorporated by reference into this prospectus or any prospectus supplement before you invest in our securities. See “Risk Factors” on page 3 of this prospectus and any risk factors contained in any applicable prospectus supplement.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is April 12, 2019.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

Unless otherwise indicated or the context otherwise requires, all references in this prospectus to “we,” “us,” “our,” “Altimmune,” the “Company” and similar designations refer to Altimmune, Inc., together with its subsidiaries.

This prospectus is part of a “shelf” registration statement that we filed with the Securities and Exchange Commission (the “SEC”). By using a shelf registration statement, we may sell, from time to time, in one or more offerings, any combination of the securities described in this prospectus.

This prospectus provides you with a general description of the securities we may offer. Each time we sell securities, we will provide a prospectus supplement that will contain specific information about the terms and manner of that offering. The accompanying prospectus supplement or information incorporated by reference into this prospectus after the date of this prospectus may also add, update or change information contained in this prospectus. Any such information that is inconsistent with this prospectus will supersede the information in this prospectus. You should read both this prospectus and the accompanying prospectus supplement together with the additional information described under the heading “Where You Can Find More Information.”

You should rely only on the information we have provided or incorporated by reference into this prospectus, any applicable prospectus supplement and any related free writing prospectus. We have not authorized anyone to provide you with different information. No dealer, salesperson or other person is authorized to give any information or to represent anything not contained in this prospectus, any applicable prospectus supplement or any related free writing prospectus.

Neither the delivery of this prospectus nor any sale made under it implies that the information in this prospectus is correct as of any date after the date of this prospectus. You should assume that the information in this prospectus, any applicable prospectus supplement or any related free writing prospectus is accurate only as of the date thereof and that any information incorporated by reference in this prospectus, any applicable prospectus supplement or any related free writing prospectus is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus, any applicable prospectus supplement or any related free writing prospectus, or any sale of a security.

This prospectus contains summaries of certain provisions contained in some of the documents described herein, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of some of the documents referred to herein have been filed, will be filed or will be incorporated by reference as exhibits to the registration statement of which this prospectus is a part, and you may obtain copies of those documents as described below under “Incorporation by Reference.”

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements, and other information with the SEC. The SEC maintains a web site at http://www.sec.gov that contains reports, proxy statements and other information about issuers, such as us, who file electronically with the SEC. Our SEC filings are available on the SEC’s Internet site. We maintain a website at http://www.altimmune.com. Information found on, or accessible through, our website is not a part of, and is not incorporated into, this prospectus supplement or the accompanying prospectus, and you should not consider it part of this prospectus supplement or part of the accompanying prospectus.

Our common stock is listed on the Nasdaq Global Market under the ticker “ALT.”

We have filed with the SEC a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Securities Act”), with respect to the securities covered by this prospectus. This prospectus, which is a part of the registration statement, does not contain all of the information set forth in the registration statement or the exhibits and schedules filed therewith. For further information with respect to us and the securities covered by this prospectus, please see the registration statement and the exhibits filed with the registration statement. A copy of the registration statement and the exhibits filed with the registration statement may be inspected without charge from the SEC as indicated above, or from us as indicated under “Incorporation by Reference.”

INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus the information that we file with the SEC. This permits us to disclose important information to you by referring to these filed documents. Any information referred to in this way is considered part of this prospectus, and any information filed with the SEC by us after the date of this prospectus will automatically be deemed to update and supersede this information. We incorporate by reference the following documents that have been filed with the SEC (other than, in each case, documents or information deemed furnished and not filed in accordance with SEC rules, including pursuant to Item 2.02 or Item 7.01 or any related exhibit furnished under Item 9.01(d) of Form 8-K, and no such information shall be deemed specifically incorporated by reference hereby):

We also incorporate by reference any future filings (other than information in such documents that is not deemed to be filed) made with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act until we file a post-effective amendment which indicates the termination of the offering of the securities made by this prospectus.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered a copy of any or all of the documents that are incorporated by reference into this prospectus supplement but not delivered with this prospectus, including exhibits that are specifically incorporated by reference in such documents. You may request a copy of such documents at no cost, by writing or telephoning us at the following address or telephone number: Altimmune, Inc., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland, Attention: Chief Financial Officer. Our telephone number is (240) 654-1450.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference herein contain forward-looking statements within the meaning of Section 27A of the Securities Act and Section 21E of the Exchange Act. Some of these statements can be identified by use of forward-looking words such as “believes,” “expects,” “anticipates,” “may,” “will,” “should,” “seeks,” “approximately,” “intends,” “plans” or “estimates,” or the negative of these words, or other comparable terminology. The discussion of financial trends, strategy, plans or intentions may also include forward-looking statements. Forward-looking statements involve risks and uncertainties that could cause actual results to differ materially from those projected, anticipated, or implied. Although it is not possible to predict or identify all such risks and uncertainties, they may include, but are not limited to, the factors discussed under “Risk Factors” in our most recent Annual Report on Form 10-K, in our Quarterly Reports on Form 10-Q, in any prospectus supplement related hereto, and in other information contained in our publicly available SEC filings and press releases.

You should not consider this list to be a complete statement of all risks and uncertainties. You are cautioned not to place undue reliance on any such forward-looking statements, which speak only as of the date such statements were first made. Except to the extent required by federal securities laws, we undertake no obligation to publicly release the result of any revisions to these forward-looking statements to reflect events or circumstances after the date hereof or to reflect the occurrence of unanticipated events.

RISK FACTORS

Investing in our securities involves a high degree of risk. Before making a decision to invest in our securities, in addition to the other information contained in this prospectus, any accompanying prospectus supplement or any related free writing prospectus, or incorporated by reference herein or therein, you should carefully consider the risks discussed under “Risk Factors” in our most recent Annual Report on Form 10-K, in our Quarterly Reports on Form 10-Q, in any prospectus supplement related hereto, and in other information contained in our publicly available SEC filings and press releases. See “Where You Can Find Additional Information.”

ALTIMMUNE, INC.

Altimmune, Inc. is a clinical stage immunotherapeutics company focused on the discovery and development of products to stimulate robust and durable immune responses for the prevention and treatment of diseases. Our product candidates are focused in two key areas for which we have unique proprietary approaches; i) our synthetic peptide technology for cancer and infectious disease, and ii) intranasal vaccines for infectious disease. Using these technologies, we have generated preclinical and clinical product candidates that potentially represent an entirely new approach to harnessing the immune system. In addition, we plan to acquire or in-license product candidates in immunotherapeutic indications that are either synergistic or complementary to our capabilities to expand our pipeline.

We were incorporated in Delaware in April 2005 and subsequently changed our name to Altimmune, Inc. in May 2017 upon the completion of our merger with PharmAthene, Inc. Our principal executive offices are located at 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878, and its telephone number is (240) 654-1450. Our Internet website is www.altimmune.com and our investor relations website is located under the “Investors” tab. The information on, or that can be accessed through, our website is not part of this prospectus supplement, and you should not rely on any such information in making the decision whether to purchase our common stock.

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USE OF PROCEEDS

Unless otherwise set forth in a prospectus supplement with respect to the proceeds from the sale of the particular securities to which such prospectus supplement relates, we intend to use the net proceeds from the sale of the offered securities for general corporate purposes, including repayment or redemption of outstanding debt or preferred stock, the possible acquisition of related businesses or assets thereof, and working capital needs.

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DESCRIPTION OF COMMON STOCK

Under our Amended and Restated Certificate of Incorporation, as amended, to which we refer as our “charter,” we are currently authorized to issue 200,000,000 shares of common stock, par value $.0001 per share. As of March 27, 2019, we had 13,451,105 shares of common stock outstanding.

Holders of our common stock are entitled to one vote for each share of common stock held of record on all matters to be voted on by stockholders, except as otherwise provided by law or in any preferred stock designation. Our bylaws specify that, except as otherwise required by law or our charter, the presence in person or by proxy of holders of a majority of the shares entitled to vote at a meeting of stockholders will be necessary, and will constitute a quorum, for the transaction of business at such meeting. Our bylaws furthermore specify that all elections of directors will be determined by a plurality of the votes and that, except as otherwise provided by law or in the charter or bylaws, any other matter will be determined by the vote of a majority of the shares which are voted with regard to it. Holders of our common stock have no conversion, preemptive or other subscription rights and there are no sinking fund or redemption provisions applicable to the common stock.

There is no cumulative voting with respect to the election of directors, with the result that the holders of more than 50% of the shares voting for the election of directors can elect all of the directors then up for election. Holders of our common stock are entitled to receive dividends when, as and if declared by our board of directors out of funds legally available therefor. In the event of our liquidation, dissolution or winding up, the holders of common stock are entitled to share in all assets remaining which are available for distribution to them after payment of liabilities and after provision has been made for each class of stock, if any, having preference over the common stock.

Transfer Agent

The transfer agent and registrar for the common stock is Continental Stock Transfer & Trust Company, New York, New York.

DESCRIPTION OF PREFERRED STOCK

Under our charter, we are currently authorized to issue 1,000,000 shares of preferred stock, par value $.0001 per share. As of the date of this prospectus, we had no shares of preferred stock outstanding.

Under our charter, our board of directors is expressly granted authority to issue shares of preferred stock, in one or more series, and to fix for each series such voting powers, full or limited, and such designations, preferences and relative, participating, optional or other special rights and such qualifications, limitations or restrictions as it may determine in the resolution or resolutions providing for the issue of such series (to which we also refer as a “preferred stock designation”) and as may be permitted by the Delaware General Corporation Law. The number of authorized shares of preferred stock may be increased or decreased (but not below the number of shares of preferred stock then outstanding) by the affirmative vote of the holders of a majority of the voting power of all of the then outstanding shares of our capital stock entitled to vote generally in the election of directors, voting together as a single class, without a separate vote of the holders of the preferred stock, or any series of preferred stock, unless a vote of any such holders is required pursuant to any preferred stock designation.

This section describes the general terms of our preferred stock to which any prospectus supplement may relate. A prospectus supplement will describe the terms relating to any preferred stock to be offered by us in greater detail, and may provide information that is different from the information described in this prospectus. If the information in the prospectus supplement with respect to the particular preferred stock being offered differs from the information in this prospectus, you should rely on the information in the prospectus supplement. A copy

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of our charter has been incorporated by reference from our filings with the SEC as an exhibit to the registration statement of which this prospectus is a part. A certificate of designations will specify the terms of the preferred stock being offered, and will be filed as an exhibit to the registration statement of which this prospectus is a part or incorporated by reference from a report that we file with the SEC.

The rights and terms relating to any new series of preferred stock could adversely affect the voting power or other rights of the holders of the common stock or could be utilized, under certain circumstances, as a method of discouraging, delaying or preventing a change in control of the Company.

The following description of our preferred stock, together with any description of our preferred stock in a related prospectus supplement, summarizes the material terms and provisions of the preferred stock that we may sell under this prospectus. We urge you to read the applicable prospectus supplement(s) related to the particular series of preferred stock that we sell under this prospectus and to the actual terms and provisions contained in our charter (certificate of designations) and bylaws, each as amended from time to time.

Terms

Our board of directors will fix the rights, preferences, privileges, qualifications and restrictions of the preferred stock of each series that we sell under this prospectus and applicable prospectus supplements in the certificate of designations relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part or incorporate by reference the form of any certificate of designations that describes the terms of the series of preferred stock we are offering in connection with the issuance of the related series of preferred stock. This description of the preferred stock in the certificate of designations and any applicable prospectus supplement may include:

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Rank

As set forth in the applicable supplement to this prospectus, shares of our preferred stock may rank, with respect to payment of distributions and rights upon our liquidation, dissolution or winding up, and allocation of our earnings and losses:

Distributions

Subject to any preferential rights of any outstanding stock or series of stock, holders of our preferred stock may be entitled to receive distributions, when and as authorized by our board of directors, out of legally available funds, and share pro rata based on the number of shares of preferred stock, common stock and other equity securities outstanding.

Voting Rights

As indicated in the applicable supplement to this prospectus, and as otherwise required under Delaware law, holders of our preferred stock may or may not have voting rights.

Liquidation Preference

As indicated in the applicable supplement to this prospectus, upon the voluntary or involuntary liquidation, dissolution or winding up of our affairs, then, before any distribution or payment shall be made to the holders of any common stock or any other class or series of stock ranking junior to the preferred stock in our distribution of assets upon any liquidation, dissolution or winding up, the holders of each series of our preferred stock may be entitled to receive, after payment or provision for payment of our debts and other liabilities, out of our assets legally available for distribution to shareholders, liquidating distributions in the amount of the liquidation preference per share (set forth in the applicable supplement to this prospectus), plus an amount, if applicable, equal to all distributions accrued and unpaid thereon (which shall not include any accumulation in respect of unpaid distributions for prior distribution periods if the preferred stock does not have a cumulative distribution). After payment of the full amount of the liquidating distributions to which they may be entitled, the holders of preferred stock may have no right or claim to any of our remaining assets. In the event that, upon our voluntary or involuntary liquidation, dissolution or winding up, the legally available assets are insufficient to pay the amount of the liquidating distributions on all of our outstanding preferred stock and the corresponding amounts payable on all of our stock of other classes or series of equity security ranking on a parity with the preferred stock in the distribution of assets upon liquidation, dissolution or winding up, then the holders of our preferred stock and all other such classes or series of equity securities may share ratably in the distribution of assets in proportion to the full liquidating distributions to which they would otherwise be respectively entitled.

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If the liquidating distributions are made in full to all holders of preferred stock, our remaining assets may be distributed among the holders of any other classes or series of equity security ranking junior to the preferred stock upon our liquidation, dissolution, or winding up, according to their respective rights and preferences and in each case according to their respective number of shares of stock.

Conversion Rights

The terms and conditions, if any, upon which shares of any series of preferred stock are convertible into, such as common stock, debt securities, warrants or units consisting of one or more of such securities will be set forth in the applicable supplement to this prospectus. These terms will include the amount and type of security into which the shares of preferred stock are convertible, the conversion price (or manner of calculation thereof), the conversion period, provisions as to whether conversion will be at the option of the holders of the preferred stock or us, the events, if any, requiring an adjustment of the conversion price and provisions, if any, affecting conversion in the event of the redemption of that preferred stock.

Redemption

If so provided in the applicable supplement to this prospectus, our preferred stock will be subject to mandatory redemption or redemption at our option, in whole or in part, in each case upon the terms, at the times and at the redemption prices set forth in such supplement to this prospectus.

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DESCRIPTION OF DEPOSITARY SHARES

The following description of the depositary shares does not purport to be complete and is subject to and qualified in its entirety by the Deposit Agreement and the depositary receipt relating to the preferred stock that is attached to the Deposit Agreement. You should read these documents as they, and not this description, define your rights as a holder of depositary shares. Forms of these documents have been filed with the SEC as an exhibit to the registration statement of which this prospectus forms a part.

General

If we elect to offer fractional interests in shares of preferred stock, we will provide for the issuance by a depositary to the public of receipts for depositary shares. Each depositary share will represent fractional interests of preferred stock. We will deposit the shares of preferred stock underlying the depositary shares under a Deposit Agreement between us and a bank or trust company selected by us. The bank or trust company must have its principal office in the United States and a combined capital and surplus of at least $50 million. The depositary receipts will evidence the depositary shares issued under the Deposit Agreement.

The Deposit Agreement will contain terms applicable to the holders of depositary shares in addition to the terms stated in the depositary receipts. Each owner of depositary shares will be entitled to all the rights and preferences of the preferred stock underlying the depositary shares in proportion to the applicable fractional interest in the underlying shares of preferred stock. The depositary will issue the depositary receipts to individuals purchasing the fractional interests in shares of the related preferred stock according to the terms of the offering described in a prospectus supplement.

Dividends and Other Distributions

The depositary will distribute all cash dividends or other cash distributions received for the preferred stock to the entitled record holders of depositary shares in proportion to the number of depositary shares that the holder owns on the relevant record date. The depositary will distribute only an amount that can be distributed without attributing to any holder of depositary shares a fraction of one cent. The depositary will add the undistributed balance to and treat it as part of the next sum received by the depositary for distribution to holders of depositary shares.

If there is a non-cash distribution, the depositary will distribute property received by it to the entitled record holders of depositary shares, in proportion, insofar as possible, to the number of depositary shares owned by the holders, unless the depositary determines, after consultation with us, that it is not feasible to make such distribution. If this occurs, the depositary may, with our approval, sell such property and distribute the net proceeds from the sale to the holders. The Deposit Agreement also will contain provisions relating to how any subscription or similar rights that we may offer to holders of the preferred stock will be available to the holders of the depositary shares.

Conversion, Exchange and Redemption

If any series of preferred stock underlying the depositary shares may be converted or exchanged, each record holder of depositary receipts will have the right or obligation to convert or exchange the depositary shares represented by the depositary receipts.

Whenever we redeem shares of preferred stock held by the depositary, the depositary will redeem, at the same time, the number of depositary shares representing the preferred stock. The depositary will redeem the depositary shares from the proceeds it receives from the corresponding redemption, in whole or in part, of the applicable series of preferred stock. The depositary will mail notice of redemption to the record holders of the depositary shares that are to be redeemed between 30 and 60 days before the date fixed for redemption. The redemption price per depositary share will be equal to the applicable fraction of the redemption price per share on

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the applicable series of preferred stock. If less than all the depositary shares are to be redeemed, the depositary will select which shares to be redeemed by lot, proportionate allocation or any other method.

After the date fixed for redemption, the depositary shares called for redemption will no longer be outstanding. When the depositary shares are no longer outstanding, all rights of the holders will end, except the right to receive money, securities or other property payable upon redemption.

Voting

When the depositary receives notice of a meeting at which the holders of the preferred stock are entitled to vote, the depositary will mail the particulars of the meeting to the record holders of the depositary shares. Each record holder of depositary shares on the record date may instruct the depositary on how to vote the shares of preferred stock underlying the holder’s depositary shares. The depositary will try, if practical, to vote the number of shares of preferred stock underlying the depositary shares according to the instructions. The depositary will abstain from voting shares of the preferred stock to the extent it does not receive specific instructions from the holders of depositary shares representing such preferred stock. We will agree to take all reasonable action requested by the depositary to enable it to vote as instructed.

Record Date

Whenever (1) any cash dividend or other cash distribution shall become payable, any distribution other than cash shall be made, or any rights, preferences or privileges shall be offered with respect to the preferred stock, or (2) the depositary shall receive notice of any meeting at which holders of preferred stock are entitled to vote or of which holders of preferred stock are entitled to notice, or of the mandatory conversion of or any election on our part to call for the redemption of any preferred stock, the depositary shall in each such instance fix a record date (which shall be the same as the record date for the preferred stock) for the determination of the holders of depositary receipts (x) who shall be entitled to receive such dividend, distribution, rights, preferences or privileges or the net proceeds of the sale thereof or (y) who shall be entitled to give instructions for the exercise of voting rights at any such meeting or to receive notice of such meeting or of such redemption or conversion, subject to the provisions of the Deposit Agreement.

Amendments

We and the depositary may agree to amend the Deposit Agreement and the depositary receipt evidencing the depositary shares. Any amendment that (a) imposes or increases certain fees, taxes or other charges payable by the holders of the depositary shares as described in the Deposit Agreement or (b) otherwise prejudices any substantial existing right of holders of depositary shares, will not take effect until 30 days after the depositary has mailed notice of the amendment to the record holders of depositary shares. Any holder of depositary shares that continues to hold its shares at the end of the 30-day period will be deemed to have agreed to the amendment.

Termination

We may direct the depositary to terminate the Deposit Agreement by mailing a notice of termination to holders of depositary shares at least 30 days prior to termination. In addition, a Deposit Agreement will automatically terminate if:

The depositary may likewise terminate the Deposit Agreement if at any time 60 days shall have expired after the depositary shall have delivered to us a written notice of its election to resign and a successor depositary

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shall not have been appointed and accepted its appointment. If any depositary receipts remain outstanding after the date of termination, the depositary thereafter will discontinue the transfer of depositary receipts, will suspend the distribution of dividends to the holders thereof, and will not give any further notices (other than notice of such termination) or perform any further acts under the Deposit Agreement except as provided below and except that the depositary will continue (1) to collect dividends on the preferred stock and any other distributions with respect thereto and (2) to deliver the preferred stock together with such dividends and distributions and the net proceeds of any sales of rights, preferences, privileges or other property, without liability for interest thereon, in exchange for depositary receipts surrendered. At any time after the expiration of two years from the date of termination, the depositary may sell the preferred stock then held by it at public or private sales, at such place or places and upon such terms as it deems proper and may thereafter hold the net proceeds of any such sale, together with any money and other property then held by it, without liability for interest thereon, for the pro rata benefit of the holders of depositary receipts which have not been surrendered.

Payment of Fees and Expenses

We will pay all fees, charges and expenses of the depositary, including the initial deposit of the preferred stock and any redemption of the preferred stock. Holders of depositary shares will pay transfer and other taxes and governmental charges and any other charges as are stated in the Deposit Agreement for their accounts.

Resignation and Removal of Depositary

At any time, the depositary may resign by delivering notice to us, and we may remove the depositary. Resignations or removals will take effect upon the appointment of a successor depositary and its acceptance of the appointment. The successor depositary must be appointed within 60 days after delivery of the notice of resignation or removal and must be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50 million.

Reports

The depositary will forward to the holders of depositary shares all reports and communications from us that are delivered to the depositary and that we are required by law, the rules of an applicable securities exchange or our amended and restated certificate of incorporation to furnish to the holders of the preferred stock. Neither we nor the depositary will be liable if the depositary is prevented or delayed by law or any circumstances beyond its control in performing its obligations under the Deposit Agreement. The Deposit Agreement limits our obligations and the depositary’s obligations to performance in good faith of the duties stated in the Deposit Agreement. Neither we nor the depositary will be obligated to prosecute or defend any legal proceeding connected with any depositary shares or preferred stock unless the holders of depositary shares requesting us to do so furnish us with satisfactory indemnity. In performing our obligations, we and the depositary may rely upon the written advice of our counsel or accountants, on any information that competent people provide to us and on documents that we believe are genuine.

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DESCRIPTION OF DEBT SECURITIES

We may issue debt securities under one or more trust indentures to be executed by us and a specified trustee. The terms of the debt securities will include those stated in the indenture and those made a part of the indenture by reference to the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”). The indentures will be qualified under the Trust Indenture Act.

The following description sets forth certain anticipated general terms and provisions of the debt securities to which an accompanying prospectus supplement may relate. The particular terms of the debt securities offered by an accompanying prospectus supplement (which terms may be different than those stated below) and the extent, if any, to which such general provisions may apply to the debt securities so offered will be described in the prospectus supplement relating to such debt securities. Accordingly, for a description of the terms of a particular issue of debt securities, investors should review both the accompanying prospectus supplement relating thereto and the following description. A form of the indenture has been filed as an exhibit to the registration statement of which this prospectus is a part.

The debt securities will be our direct obligations and may be either senior debt securities or subordinated debt securities. The indebtedness represented by subordinated securities will be subordinated in right of payment to the prior payment in full of our senior debt (as defined in the applicable indenture).

Except as set forth in the applicable indenture and described in an accompanying prospectus supplement relating thereto, the debt securities may be issued without limit as to aggregate principal amount, in one or more series, secured or unsecured, in each case as established from time to time in or pursuant to authority granted by a resolution of the board of trustees or as established in the applicable indenture. All debt securities of one series need not be issued at the same time and, unless otherwise provided, a series may be reopened, without the consent of the holders of the debt securities of such series, for issuance of additional debt securities of such series.

The accompanying prospectus supplement relating to any series of debt securities being offered will contain their specific terms, including, without limitation:

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The debt securities may provide for less than the entire principal amount thereof to be payable upon declaration of acceleration of the maturity thereof. Special federal income tax, accounting and other considerations applicable to debt securities will be described in the accompanying prospectus supplement.

The applicable indenture may contain provisions that would limit our ability to incur indebtedness or that would afford holders of debt securities protection in the event of a highly leveraged or similar transaction involving us or in the event of a change of control.

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Investors should review the accompanying prospectus supplement for information with respect to any deletions from, modifications of or additions to the events of default or covenants that are described below, including any addition of a covenant or other provision providing event risk or similar protection.

Merger, Consolidation or Sale

The applicable indenture will provide that we may consolidate with or merge into, or convey, transfer or lease all or substantially all of our properties and assets to any other person (as defined therein), provided that:

Covenants

The applicable indenture will contain covenants requiring us to take certain actions and prohibiting us from taking certain actions. The covenants with respect to any series of debt securities will be described in the accompanying prospectus supplement.

Events of Default, Notice and Waiver

Each indenture will describe specific “events of default” with respect to a series of debt securities issued under the indenture. These “events of default” are likely to include (with grace and cure periods):

If an event of default resulting from certain events of bankruptcy described in the indenture occurs, all outstanding debt securities of that series will become due and payable immediately. If any other event of default under any indenture with respect to debt securities of any series at the time outstanding occurs and is continuing, then the applicable trustee or the holders of not less than 25% of the principal amount of the outstanding debt securities of that series may declare the principal amount (or, if the debt securities of that series are original issue discount securities or indexed securities, such portion of the principal amount as may be specified in the terms thereof) of all the debt securities of that series to be due and payable immediately by written notice thereof to us (and to the applicable trustee if given by the holders). However, at any time after such a declaration of acceleration with respect to debt securities of such series (or of all debt securities then outstanding under any indenture, as the case may be) has been made, the holders of not less than a majority in principal amount of outstanding debt securities of such series (or of all debt securities then outstanding under the applicable indenture, as the case may be) may rescind and annul such declaration and its consequences if:

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Each indenture also will provide that the holders of not less than a majority in principal amount of the outstanding debt securities of any series (or of all debt securities then outstanding under the applicable indenture, as the case may be) may waive any past default with respect to the series and its consequences, except a:

Each trustee will be required to give notice to the holders of debt securities within a certain number of days of a default under the applicable indenture unless the default has been cured or waived; provided, however, that the trustee may withhold notice to the holders of any series of debt securities of any default with respect to the series (except a default in the payment of the principal of (or premium, if any) or interest on any debt security of the series or in the payment of any sinking fund installment in respect of any debt security of the series) if specified responsible officers of the trustee consider withholding the notice to be in the interest of the holders.

Each indenture will prohibit the holders of debt securities of any series from instituting any proceedings, judicial or otherwise, with respect to the indenture or for any remedy thereunder, except in the case of failure of the applicable trustee, for a certain period of time after the trustee has received a written request to institute proceedings in respect of an event of default from the holders of not less than a majority in principal amount of the outstanding debt securities of such series, as well as the furnishing of indemnity reasonably satisfactory to it. This provision will not prevent any holder of debt securities from instituting a suit to enforce the payment of the principal of (and premium, if any) and interest on the debt securities at the respective due dates thereof.

Subject to the indenture, no trustee will be under any obligation to exercise any of its rights or powers under an indenture at the request or direction of any holders of any series of debt securities then outstanding, unless the holders furnish the trustee thereunder reasonable security or indemnity. The holders of not less than a majority in principal amount of the outstanding debt securities of any series (or of all debt securities then outstanding under an indenture, as the case may be) will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee, or of exercising any trust or power conferred upon the trustee. However, a trustee may refuse to follow any direction, which is in conflict with any law or the applicable indenture, which may involve the trustee in personal liability or which may be unduly prejudicial to the holders of debt securities of such series not joining therein.

Within a certain period of time of the close of each fiscal year, we will be required to deliver to each trustee, a certificate, signed by one of several specified officers, stating whether or not the officer has knowledge of any default under the applicable indenture and, if so, specifying each default and the nature and status thereof.

Modification of the Indenture

The indenture will likely provide that it may be modified or amended, with the consent of the holders of not less than a majority in principal amount of each series of the outstanding debt securities issued under the indenture affected by the modification or amendment, provided that no modification or amendment may, without the consent of each affected holder of the debt securities:

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A record date may be set for any act of the holders with respect to consenting to any amendment.

The holders of not less than a majority in principal amount of the outstanding debt securities of each series affected thereby will have the right to waive our compliance with certain covenants in the indenture. Each indenture will contain provisions for convening meetings of the holders of debt securities of a series to take permitted action. Under certain circumstances, we and the trustee may make modifications and amendments to an indenture without the consent of any holders of outstanding debt securities.

Redemption of Debt Securities

The debt securities may be subject to optional or mandatory redemption on terms and conditions described in the applicable accompanying prospectus supplement.

Conversion of Debt Securities

The terms and conditions, if any, upon which any debt securities are convertible or into our common stock or preferred stock will be set forth in the applicable accompanying prospectus supplement. The terms will include:

Subordination

Upon any distribution to our creditors in a liquidation, dissolution or reorganization, the payment of the principal of and interest on any subordinated securities will be subordinated to the extent provided in the applicable indenture to the prior payment in full of all senior securities. No payment of principal or interest will be permitted to be made on subordinated securities at any time if any payment default or any other default which permits accelerations exists. After all senior securities are paid in full and until the subordinated securities are paid in full, holders of subordinated securities will be subrogated to the right of holders of senior securities to the extent that distributions otherwise payable to holders of subordinated securities have been applied to the payment of senior securities. By reason of any subordination, in the event of a distribution of assets upon our insolvency, some of our general creditors may recover more, ratably, than holders of subordinated securities. The accompanying prospectus supplement or the information incorporated herein by reference will contain the approximate amount of senior securities outstanding as of the end of our most recent fiscal quarter.

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Global Debt Securities

The debt securities of a series may be issued in whole or in part in global form. The global securities will be deposited with a depositary, or with a nominee for a depositary, identified in the accompanying prospectus supplement. In this case, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the aggregate principal amount of outstanding debt securities of the series to be represented by the global security or securities. Unless and until it is exchanged in whole or in part for debt securities in definitive form, a global security may not be transferred except as a whole by the depositary for the global security to a nominee of the depositary or by a nominee of the depositary to the depositary or another nominee of the depositary or by the depositary or any nominee to a successor of the depositary or a nominee of the successor.

The specific material terms of the depositary arrangement with respect to any portion of a series of debt securities to be represented by a global security will be described in the applicable accompanying prospectus supplement. We anticipate that the following provisions will apply to all depositary arrangements.

Upon the issuance of a global security, the depositary for the global security will credit, on its book-entry registration and transfer system, the respective principal amounts of the debt securities represented by the global security to the accounts of persons or participants that have accounts with the depositary. The accounts to be credited will be designated by any underwriters or agents participating in the distribution of the debt securities. Ownership of beneficial interests in a global security will be limited to participants or persons that may hold interests through participants. Ownership of beneficial interests in the global security will be shown on, and the transfer of that ownership will be effected only through, records maintained by the depositary for the global security, with respect to interests of participants, or by participants or persons that hold through participants, with respect to interests of persons other than participants. So long as the depositary for a global security, or its nominee, is the registered owner of the global security, the depositary or the nominee, as the case may be, will be considered the sole owner or holder of the debt securities represented by the global security for all purposes under the indenture; provided, however, that for purposes of obtaining any consents or directions required to be given by the holders of the debt securities, we, the trustee and our agents will treat a person as the holder of the principal amount of debt securities as specified in a written statement of the depositary. Except as set forth herein or otherwise provided in the accompanying prospectus supplement, owners of beneficial interests in a global security will not be entitled to have the debt securities represented by the global security registered in their names, will not receive physical delivery of the debt securities in definitive form and will not be considered the owners or holders thereof under the indenture.

Principal, premium, if any, and interest payments on debt securities represented by a global security registered in the name of a depositary or its nominee will be made to the depositary or its nominee, as the case may be, as the registered owner of the global security. Neither we, the trustee nor any paying agent for the debt securities will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global security or for maintaining, supervising or reviewing any records relating to the beneficial ownership interests.

We expect that the depositary for any debt securities represented by a global security, upon receipt of any payment of principal, premium, if any, or interest will immediately credit participants’ accounts with payments in amounts proportionate to their respective beneficial interests in the principal amount of the global security as shown on the records of the depositary. We also expect that payments by participants will be governed by standing instructions and customary practices, as is now the case with the securities held for the accounts of customers registered in “street names” and will be the responsibility of the participants.

If the depositary for any debt securities represented by a global security is at any time unwilling or unable to continue as depositary and a successor depositary is not appointed by us within the period of time set forth in the indenture, we will issue the debt securities in definitive form in exchange for the global security. In addition, we

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may at any time, and in our sole discretion, determine not to have any of the debt securities of a series represented by one or more global securities and, in that event, will issue debt securities of the series in definitive form in exchange for all of the global security or securities representing the debt securities.

Governing Law

The indenture and the debt securities will be governed by and construed in accordance with the internal laws of the State of New York.

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DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus. While the terms we have summarized below will apply generally to any warrants that we may offer under this prospectus, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The terms of any warrants offered under a prospectus supplement may differ from the terms described below.

We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from another report that we file with the SEC, the form of warrant agreement, which may include a form of warrant certificate, that describes the terms of the particular series of warrants we are offering. The following summary of material provisions of the warrants and the warrant agreements are subject to all the provisions of the warrant agreement and warrant certificate applicable to a particular series of warrants. We urge you to read the applicable prospectus supplements related to the particular series of warrants that we sell under this prospectus, as well as the complete warrant agreements and/or warrant certificates that contain the terms of the warrants.

General

We will describe in the applicable prospectus supplement the terms relating to warrants being offered, which may include:

Before exercising their warrants, holders of warrants will likely not have any of the rights of holders of the securities purchasable upon such exercise, including, in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or payments upon our liquidation, dissolution or winding up of our affairs or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise

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specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to the specified time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

Holders of the warrants may exercise the warrants by delivering the warrant certificate representing the warrants to be exercised together with specified information, and paying the required amount to the warrant agent in immediately available funds, as provided in the applicable prospectus supplement. We intend to set forth in any warrant agreement and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

Upon receipt of the required payment and any warrant certificate or other form required for exercise properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant or warrant certificate are exercised, then we will issue a new warrant or warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

We may issue stock purchase contracts, representing contracts obligating holders to purchase from us, and requiring us to sell to the holders, a specified number of shares of common stock at a future date or dates. The price per share of common stock may be fixed at the time the stock purchase contracts are issued or may be determined by reference to a specific formula set forth in the stock purchase contracts. The stock purchase contracts may be issued separately or as a part of units, or stock purchase units, consisting of a stock purchase contract and either (x) senior debt securities, senior subordinated debt securities, subordinated debt securities or junior subordinated debt securities, or (y) debt obligations of third parties, including U.S. Treasury securities, in each case, securing the holder’s obligations to purchase the common stock under the stock purchase contracts. The stock purchase contracts may require us to make periodic payments to the holders of the stock purchase contracts or vice versa, and such payments may be unsecured or prefunded on some basis. The stock purchase contracts may require holders to secure their obligations thereunder in a specified manner and in certain circumstances we may deliver newly issued prepaid stock purchase contracts, or prepaid securities, upon release to a holder of any collateral securing such holder’s obligations under the original stock purchase contract.

The applicable prospectus supplement will describe the terms of any stock purchase contracts or stock purchase units and, if applicable, prepaid securities. The description in the prospectus supplement will not purport to be complete and will be qualified in its entirety by reference to the stock purchase contracts, the collateral arrangements and depositary arrangements, if applicable, relating to such stock purchase contracts or stock purchase units and, if applicable, the prepaid securities and the document pursuant to which such prepaid securities will be issued.

PLAN OF DISTRIBUTION

Altimmune may sell common stock, preferred stock, depositary shares, debt securities, warrants, stock purchase contracts or stock purchase units in one or more of the following ways from time to time:

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Altimmune may also sell it securities, including shares of its common stock, in one or more of the following transactions: (i) block transactions (which may involve crosses) in which a broker-dealer may sell all or a portion of such shares as agent, but may position and resell all or a portion of the block as principal to facilitate the transaction; (ii) purchases by any such broker-dealer as principal, and resale by such broker-dealer for its own account pursuant to an accompanying prospectus supplement; (iii) a special offering, an exchange distribution or a secondary distribution in accordance with applicable Nasdaq Global Market or other stock exchange, quotation system or over-the-counter market rules; (iv) ordinary brokerage transactions and transactions in which any such broker-dealer solicits purchasers; (v) sales “at the market” to or through a market maker or into an existing trading market, on an exchange or otherwise, for such shares; and (vi) sales in other ways not involving market makers or established trading markets.

For each offering of securities, the applicable prospectus supplement or other offering materials relating to the offering will set forth the terms of such offering, including:

Any initial public offering prices, discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

If underwriters are used in the sale, the underwriters will acquire the offered securities for their own account and may resell them from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The offered securities may be offered either to the public through underwriting syndicates represented by one or more managing underwriters or by one or more underwriters without a syndicate. Unless otherwise set forth in a prospectus supplement, the obligations of the underwriters to purchase any series of securities will be subject to certain conditions precedent, and the underwriters will be obligated to purchase all of such series of securities, if any are purchased.

In connection with underwritten offerings of the offered securities and in accordance with applicable law and industry practice, underwriters may over-allot or effect transactions that stabilize, maintain or otherwise affect the market price of the offered securities at levels above those that might otherwise prevail in the open market, including by entering stabilizing bids, effecting syndicate covering transactions or imposing penalty bids, each of which is described below:

These transactions may be effected on the New York Stock Exchange, in the over-the-counter market, or otherwise. Underwriters are not required to engage in any of these activities, or to continue such activities if commenced.

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If a dealer is used in the sale, Altimmune will sell such offered securities to the dealer, as principal. The dealer may then resell the offered securities to the public at varying prices to be determined by that dealer at the time for resale. The names of the dealers and the terms of the transaction will be set forth in the prospectus supplement relating to that transaction.

Offered securities may be sold directly by Altimmune to one or more institutional purchasers, or through agents designated by Altimmune from time to time, at a fixed price or prices, which may be changed, or at varying prices determined at the time of sale. Any agent involved in the offer or sale of the offered securities in respect of which this prospectus is delivered will be named, and any commissions payable by Altimmune to such agent will be set forth, in the prospectus supplement relating to that offering. Unless otherwise indicated in such prospectus supplement, any such agent will be acting on a best efforts basis for the period of its appointment.

Underwriters, dealers and agents may be entitled under agreements entered into with us to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, or to contribution with respect to payments that the underwriters, dealers or agents may be required to make in respect thereof. Underwriters, dealers and agents may be customers of, engage in transactions with, or perform services for us and our affiliates in the ordinary course of business.

Other than our common stock, which is listed on the Nasdaq Global Market, each of the securities issued hereunder will be a new issue of securities, will have no prior trading market, and may or may not be listed on a national securities exchange. Any common stock sold pursuant to a prospectus supplement will be listed on the Nasdaq Global Market, subject to official notice of issuance. Any underwriters to whom Altimmune sells securities for public offering and sale may make a market in the securities, but such underwriters will not be obligated to do so and may discontinue any market making at any time without notice. We cannot assure you that there will be a market for the offered securities.

LEGAL MATTERS

Unless otherwise stated in an accompanying prospectus supplement, Morrison & Foerster LLP, Boston, Massachusetts, will provide us with an opinion as to the legality of the securities offered under this prospectus. Counsel representing any underwriters, dealers or agents will be named in the applicable prospectus supplement.

EXPERTS

The consolidated financial statements of Altimmune, Inc. appearing in Altimmune, Inc.’s Annual Report (Form 10-K) for the year ended December 31, 2018, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance on such report given on the authority of said firm as experts in accounting and auditing.

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                 Shares of Common Stock

Pre-Funded Warrants to Purchase                  Shares of Common Stock

PROSPECTUS SUPPLEMENT

Joint Book-Running Managers

Jefferies

Evercore ISI

Piper Sandler

Co-Manager

Roth Capital Partners

                , 2020