As filed with the Securities and Exchange Commission on May 13, 2026

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Registration No. 333-

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

Under

The Securities Act of 1933

ALTIMMUNE, INC.

(Exact name of registrant as specified in its charter)

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ALTIMMUNE, INC.

2019 EMPLOYEE STOCK PURCHASE PLAN

(Full Title of the Plan)

Jerome Durso

Chief Executive Officer

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, Maryland

Telephone: (240) 654-1450

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Joseph Theis, Jr.

Justin Anslow

Goodwin Procter LLP

100 Northern Ave

Boston, MA 02210

Telephone: (617) 570-1000

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ¨

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EXPLANATORY NOTE

This Registration Statement on Form S-8 registers an additional 705,327 shares of the Registrant’s common stock, $0.0001 par value per share (“Common Stock”) under the Altimmune, Inc. 2019 Employee Stock Purchase Plan, as amended (the “2019 ESPP”). The additional shares are of the same class as other securities relating to the 2019 ESPP for which the Registrant’s registration statement filed on Form S-8 filed with the Securities and Exchange Commission on April 4, 2019 (Registration No. 333-230722) (the “Registration Statement”), is effective. The information contained in the Registration Statement is hereby incorporated by reference pursuant to General Instruction E.

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Part II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 8. Exhibits.

See the Exhibit Index below for a list of exhibits filed as part of this registration statement on Form S-8, which Exhibit Index is incorporated herein by reference.

EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Gaithersburg, State of Maryland, on the 13th day of May, 2026.

POWER OF ATTORNEY AND SIGNATURES

KNOW ALL BY THESE PRESENT, that each individual whose signature appears below hereby constitutes and appoints each of Jerome Durso and Gregory Weaver as such person’s true and lawful attorney-in-fact and agent with full power of substitution and resubstitution, for such person in such person’s name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement on Form S-8, and to file the same, with all exhibits thereto, and all documents in connection therewith, with the Securities and Exchange Commission granting unto each said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as such person might or could do in person, hereby ratifying and confirming all that any said attorney-in-fact and agent, or any substitute or substitutes of any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed by the following person in the capacities and on the date indicated.

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Exhibit 5.1

May 13, 2026

Altimmune, Inc.

910 Clopper Road Suite 201S

Gaithersburg, Maryland

Ladies and Gentlemen:

We have acted as your counsel in connection with your filing of a Registration Statement on Form S-8 (the “Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), on or about the date hereof relating to an aggregate of 705,327 shares (the “Shares”) of Common Stock, par value $0.0001 per share (“Common Stock”), of Altimmune, Inc., a Delaware corporation (the “Company”), that may be issued pursuant to the Company’s 2019 Employee Stock Purchase Plan, as amended (the “Plan”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

For purposes of the opinion set forth below, we have assumed that, at the time Shares are issued, the total number of then unissued Shares, when added to the number of shares of Common Stock issued, subscribed for, or otherwise committed to be issued, does not exceed the number of shares of Common Stock authorized by the Company’s certificate of incorporation.

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The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when delivered against payment therefor in accordance with the terms of the Plan, will be validly issued, fully paid and nonassessable.

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This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,

/s/ GOODWIN PROCTER LLP

GOODWIN PROCTER LLP

Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Registration Statement (Form S-8) pertaining to the Altimmune, Inc. 2019 Employee Stock Purchase Plan, as amended, of our report dated March 6, 2026, with respect to the consolidated financial statements of Altimmune, Inc. included in its Annual Report (Form 10-K) for the year ended December 31, 2025, filed with the Securities and Exchange Commission.

/s/ Ernst & Young LLP

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Iselin, New Jersey

May 13, 2026