<?xml version="1.0" encoding="us-ascii"?>
<!--XBRL document created with Toppan Merrill Bridge  10.1.0.3 -->
<!--Based on XBRL 2.1-->
<!--Created on: 11/10/2022 4:20:58 PM-->
<!--Modified on: 11/10/2022 4:20:58 PM-->
<xsd:schema targetNamespace="http://www.altimmune.com/20220930" attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:us-gaap="http://fasb.org/us-gaap/2021-01-31" xmlns:srt-types="http://fasb.org/srt-types/2021-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:naics="http://xbrl.sec.gov/naics/2021" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:srt="http://fasb.org/srt/2021-01-31" xmlns:currency="http://xbrl.sec.gov/currency/2021" xmlns:exch="http://xbrl.sec.gov/exch/2021" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:sic="http://xbrl.sec.gov/sic/2021" xmlns:country="http://xbrl.sec.gov/country/2021" xmlns:us-types="http://fasb.org/us-types/2021-01-31" xmlns:stpr="http://xbrl.sec.gov/stpr/2021" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2020-01-21" xmlns:alt="http://www.altimmune.com/20220930" xmlns:enum2="http://xbrl.org/2020/extensible-enumerations-2.0" xmlns:us-roles="http://fasb.org/us-roles/2021-01-31" xmlns:ref="http://www.xbrl.org/2006/ref" xmlns:xl="http://www.xbrl.org/2003/XLink" xmlns:srt-roles="http://fasb.org/srt-roles/2021-01-31" xmlns:utr="http://www.xbrl.org/2009/utr" xmlns:negated="http://www.xbrl.org/2009/role/negated" xmlns:attributeFormDefault="unqualified" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:deprecated="http://www.xbrl.org/2009/role/deprecated" xmlns:net="http://www.xbrl.org/2009/role/net" xmlns:dei="http://xbrl.sec.gov/dei/2021q4">
  <xsd:annotation>
    <xsd:appinfo>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStandardAndCustomAxisDomainDefaults" id="DisclosureStandardAndCustomAxisDomainDefaults">
        <link:definition>99900 - Disclosure - Standard And Custom Axis Domain Defaults</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/StatementConsolidatedBalanceSheets" id="StatementConsolidatedBalanceSheets">
        <link:definition>00100 - Statement - CONSOLIDATED BALANCE SHEETS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss" id="StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss">
        <link:definition>00200 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/StatementConsolidatedStatementsOfCashFlows" id="StatementConsolidatedStatementsOfCashFlows">
        <link:definition>00400 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureFairValueMeasurementsScheduleOfShortTermInvestmentsDetail" id="DisclosureFairValueMeasurementsScheduleOfShortTermInvestmentsDetail">
        <link:definition>40302 - Disclosure - Fair Value Measurements - Schedule of Short Term Investments (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureAccruedExpensesScheduleOfAccruedExpensesAndOtherCurrentLiabilitiesDetail" id="DisclosureAccruedExpensesScheduleOfAccruedExpensesAndOtherCurrentLiabilitiesDetail">
        <link:definition>40501 - Disclosure - Accrued Expenses - Schedule of Accrued Expenses and Other Current Liabilities (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOtherLongTermLiabilitiesSummaryOfOtherLongTermLiabilitiesDetail" id="DisclosureOtherLongTermLiabilitiesSummaryOfOtherLongTermLiabilitiesDetail">
        <link:definition>40701 - Disclosure - Other Long-Term Liabilities - Summary of Other Long-Term Liabilities (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquity" id="StatementConsolidatedStatementsOfChangesInStockholdersEquity">
        <link:definition>00300 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureFairValueMeasurementsScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisDetail" id="DisclosureFairValueMeasurementsScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisDetail">
        <link:definition>40301 - Disclosure - Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureFairValueMeasurementsAdditionalInformationDetail" id="DisclosureFairValueMeasurementsAdditionalInformationDetail">
        <link:definition>40303 - Disclosure - Fair Value Measurements - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureContingentConsiderationAdditionalInformationDetail" id="DisclosureContingentConsiderationAdditionalInformationDetail">
        <link:definition>40601 - Disclosure - Contingent Consideration - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureCommonStockAdditionalInformationDetail" id="DisclosureCommonStockAdditionalInformationDetail">
        <link:definition>40801 - Disclosure - Common Stock - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureWarrantsSummaryOfWarrantActivityDetail" id="DisclosureWarrantsSummaryOfWarrantActivityDetail">
        <link:definition>40901 - Disclosure - Warrants - Summary of Warrant Activity (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureWarrantsAdditionalInformationDetail" id="DisclosureWarrantsAdditionalInformationDetail">
        <link:definition>40902 - Disclosure - Warrants - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStockBasedCompensationAdditionalInformationDetail" id="DisclosureStockBasedCompensationAdditionalInformationDetail">
        <link:definition>41001 - Disclosure - Stock-Based Compensation - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStockBasedCompensationScheduleOfStockBasedCompensationExpenseDetail" id="DisclosureStockBasedCompensationScheduleOfStockBasedCompensationExpenseDetail">
        <link:definition>41003 - Disclosure - Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureUsGovernmentContractsAndGrantsAdditionalInformationDetail" id="DisclosureUsGovernmentContractsAndGrantsAdditionalInformationDetail">
        <link:definition>41101 - Disclosure - U.S. Government Contracts and Grants - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureNetLossPerShareScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareDetail" id="DisclosureNetLossPerShareScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareDetail">
        <link:definition>41301 - Disclosure - Net Loss Per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails" id="DisclosureCommitmentsAndContingenciesAdditionalInformationDetails">
        <link:definition>41401 - Disclosure - Commitments and Contingencies - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DocumentDocumentAndEntityInformation" id="DocumentDocumentAndEntityInformation">
        <link:definition>00090 - Document - Document and Entity Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/StatementConsolidatedBalanceSheetsParenthetical" id="StatementConsolidatedBalanceSheetsParenthetical">
        <link:definition>00105 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureNatureOfBusinessAndBasisOfPresentation" id="DisclosureNatureOfBusinessAndBasisOfPresentation">
        <link:definition>10101 - Disclosure - Nature of Business and Basis of Presentation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureSummaryOfSignificantAccountingPolicies" id="DisclosureSummaryOfSignificantAccountingPolicies">
        <link:definition>10201 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureFairValueMeasurements" id="DisclosureFairValueMeasurements">
        <link:definition>10301 - Disclosure - Fair Value Measurements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOperatingLeases" id="DisclosureOperatingLeases">
        <link:definition>10401 - Disclosure - Operating Leases</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureAccruedExpenses" id="DisclosureAccruedExpenses">
        <link:definition>10501 - Disclosure - Accrued Expenses</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureContingentConsideration" id="DisclosureContingentConsideration">
        <link:definition>10601 - Disclosure - Contingent Consideration</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOtherLongTermLiabilities" id="DisclosureOtherLongTermLiabilities">
        <link:definition>10701 - Disclosure - Other Long-Term Liabilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureCommonStock" id="DisclosureCommonStock">
        <link:definition>10801 - Disclosure - Common Stock</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureWarrants" id="DisclosureWarrants">
        <link:definition>10901 - Disclosure - Warrants</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStockBasedCompensation" id="DisclosureStockBasedCompensation">
        <link:definition>11001 - Disclosure - Stock-Based Compensation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureUsGovernmentContractsAndGrants" id="DisclosureUsGovernmentContractsAndGrants">
        <link:definition>11101 - Disclosure - U.S. Government Contracts and Grants</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureIncomeTaxes" id="DisclosureIncomeTaxes">
        <link:definition>11201 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureNetLossPerShare" id="DisclosureNetLossPerShare">
        <link:definition>11301 - Disclosure - Net Loss Per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureCommitmentsAndContingencies" id="DisclosureCommitmentsAndContingencies">
        <link:definition>11401 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies" id="DisclosureSummaryOfSignificantAccountingPoliciesPolicies">
        <link:definition>20202 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureFairValueMeasurementsTables" id="DisclosureFairValueMeasurementsTables">
        <link:definition>30303 - Disclosure - Fair Value Measurements (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOperatingLeasesTables" id="DisclosureOperatingLeasesTables">
        <link:definition>30403 - Disclosure - Operating Leases (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureAccruedExpensesTables" id="DisclosureAccruedExpensesTables">
        <link:definition>30503 - Disclosure - Accrued Expenses (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureContingentConsiderationTables" id="DisclosureContingentConsiderationTables">
        <link:definition>30603 - Disclosure - Contingent Consideration (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOtherLongTermLiabilitiesTables" id="DisclosureOtherLongTermLiabilitiesTables">
        <link:definition>30703 - Disclosure - Other Long-Term Liabilities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureWarrantsTables" id="DisclosureWarrantsTables">
        <link:definition>30903 - Disclosure - Warrants (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStockBasedCompensationTables" id="DisclosureStockBasedCompensationTables">
        <link:definition>31003 - Disclosure - Stock-Based Compensation (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureNetLossPerShareTables" id="DisclosureNetLossPerShareTables">
        <link:definition>31303 - Disclosure - Net Loss Per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOperatingLeasesAdditionalInformationDetail" id="DisclosureOperatingLeasesAdditionalInformationDetail">
        <link:definition>40401 - Disclosure - Operating Leases - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureOperatingLeasesSummaryOfSupplementalOtherInformationRelatedToOperatingLeasesBalanceSheetDetail" id="DisclosureOperatingLeasesSummaryOfSupplementalOtherInformationRelatedToOperatingLeasesBalanceSheetDetail">
        <link:definition>40402 - Disclosure - Operating Leases - Summary of Supplemental other Information Related to Operating Leases Balance Sheet (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureContingentConsiderationSummaryOfFairValueOfContingentConsiderationDetail" id="DisclosureContingentConsiderationSummaryOfFairValueOfContingentConsiderationDetail">
        <link:definition>40602 - Disclosure - Contingent Consideration - Summary of Fair Value of Contingent Consideration (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureStockBasedCompensationScheduleOfInformationRelatedToStockOptionsOutstandingDetail" id="DisclosureStockBasedCompensationScheduleOfInformationRelatedToStockOptionsOutstandingDetail">
        <link:definition>41002 - Disclosure - Stock-Based Compensation - Schedule of Information Related to Stock Options Outstanding (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.altimmune.com/role/DisclosureIncomeTaxesAdditionalInformationDetail" id="DisclosureIncomeTaxesAdditionalInformationDetail">
        <link:definition>41201 - Disclosure - Income Taxes - Additional Information (Detail)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:linkbaseRef xlink:type="simple" xlink:href="alt-20220930_pre.xml" xlink:title="Presentation Links, all" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
      <link:linkbaseRef xlink:type="simple" xlink:href="alt-20220930_cal.xml" xlink:title="Calculation Links, all" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
      <link:linkbaseRef xlink:type="simple" xlink:href="alt-20220930_def.xml" xlink:title="Definition Links, all" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
      <link:linkbaseRef xlink:type="simple" xlink:href="alt-20220930_lab.xml" xlink:title="Label Links, all" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:import namespace="http://xbrl.sec.gov/dei/2021q4" schemaLocation="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" />
  <xsd:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" />
  <xsd:import namespace="http://www.xbrl.org/dtr/type/2020-01-21" schemaLocation="https://www.xbrl.org/dtr/type/2020-01-21/types.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2009/role/negated" schemaLocation="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd" />
  <xsd:import namespace="http://www.xbrl.org/2009/role/net" schemaLocation="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd" />
  <xsd:import namespace="http://fasb.org/us-roles/2021-01-31" schemaLocation="https://xbrl.fasb.org/us-gaap/2021/elts/us-roles-2021-01-31.xsd" />
  <xsd:import namespace="http://fasb.org/srt/2021-01-31" schemaLocation="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd" />
  <xsd:import namespace="http://fasb.org/us-gaap/2021-01-31" schemaLocation="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd" />
  <xsd:element name="AccruedExpensesAndOtherCurrentLiabilities" id="alt_AccruedExpensesAndOtherCurrentLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="RetirementOfCommonSockInExchangeForCommonStockWarrantShares" id="alt_RetirementOfCommonSockInExchangeForCommonStockWarrantShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="RetirementOfCommonSockInExchangeForCommonStockWarrant" id="alt_RetirementOfCommonSockInExchangeForCommonStockWarrant" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="IssuanceOfCommonStockWarrantInExchangeForRetirementOfCommonStock" id="alt_IssuanceOfCommonStockWarrantInExchangeForRetirementOfCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="StockIssuedDuringPeriodShareExerciseOfWarrants" id="alt_StockIssuedDuringPeriodShareExerciseOfWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="StockIssuedDuringPeriodValueExerciseOfWarrants" id="alt_StockIssuedDuringPeriodValueExerciseOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="PaymentsOfDeferredOfferingCosts" id="alt_PaymentsOfDeferredOfferingCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="CommonStockTextBlock" id="alt_CommonStockTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="USGovernmentContractsAndGrantsTextBlock" id="alt_USGovernmentContractsAndGrantsTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="USGovernmentContractsAndGrantsDisclosuresAbstract" id="alt_USGovernmentContractsAndGrantsDisclosuresAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ScheduleOfSupplementalBalanceSheetInformationRelatedToOperatingLeasesTableTextBlock" id="alt_ScheduleOfSupplementalBalanceSheetInformationRelatedToOperatingLeasesTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommercialPaperAndCorporateDebtSecuritiesMember" id="alt_CommercialPaperAndCorporateDebtSecuritiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AssetsOrLiabilitiesMeasuredAtFairValueOnNonRecurringBasis" id="alt_AssetsOrLiabilitiesMeasuredAtFairValueOnNonRecurringBasis" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="AccruedResearchAndDevelopment" id="alt_AccruedResearchAndDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="SpitfirePharmaIncMember" id="alt_SpitfirePharmaIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="InvestigationalNewDrugApplicationMilestoneMember" id="alt_InvestigationalNewDrugApplicationMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="InvestigationalNewDrugApplicationAndRegulatoryMilestonesMember" id="alt_InvestigationalNewDrugApplicationAndRegulatoryMilestonesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SalesMilestoneMember" id="alt_SalesMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="RegulatoryMilestoneMember" id="alt_RegulatoryMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="BusinessCombinationContingentConsiderationPaymentTerm" id="alt_BusinessCombinationContingentConsiderationPaymentTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="BusinessAcquisitionTermOfLicenseAgreement" id="alt_BusinessAcquisitionTermOfLicenseAgreement" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NumberOfConsecutiveTradingDays" id="alt_NumberOfConsecutiveTradingDays" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EconomicConditionalIncentiveGrants" id="alt_EconomicConditionalIncentiveGrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="instant" />
  <xsd:element name="ClassOfWarrantOrRightIssuancePricePerWarrantOrRight" id="alt_ClassOfWarrantOrRightIssuancePricePerWarrantOrRight" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="OwnershipPercentageOnExerciseOfWarrantsWithoutNoticeMinimum" id="alt_OwnershipPercentageOnExerciseOfWarrantsWithoutNoticeMinimum" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="OwnershipPercentageOnExerciseOfWarrantsWithNoticeMaximum" id="alt_OwnershipPercentageOnExerciseOfWarrantsWithNoticeMaximum" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NoticePeriodToIncreaseOwnershipPercentageOnExerciseOfWarrants" id="alt_NoticePeriodToIncreaseOwnershipPercentageOnExerciseOfWarrants" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" id="alt_ChangesInEquityAndComprehensiveIncomeLineItemsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PreFundedWarrantsMember" id="alt_PreFundedWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="EquityDistributionAgreementMember" id="alt_EquityDistributionAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AtTheMarketOfferingMember" id="alt_AtTheMarketOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PiperSandlerCompanyAndEvercoreGroupLimitedLiabilityCompanyAndBRileySecuritiesIncMember" id="alt_PiperSandlerCompanyAndEvercoreGroupLimitedLiabilityCompanyAndBRileySecuritiesIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ExchangeWarrantsMember" id="alt_ExchangeWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="AggregateOfferingPrice" id="alt_AggregateOfferingPrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="StockExchangedForPreFundedWarrants" id="alt_StockExchangedForPreFundedWarrants" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NumberOfWarrantsExercised" id="alt_NumberOfWarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="NumberOfWarrantsExercisedForConversionOfSecurities" id="alt_NumberOfWarrantsExercisedForConversionOfSecurities" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="TwoThousandAndNineteenEmployeeStockPurchasePlanMember" id="alt_TwoThousandAndNineteenEmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsUnvestedOutstandingWeightedAverageExercisePrice" id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsUnvestedOutstandingWeightedAverageExercisePrice" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsUnvestedOutstandingWeightedAverageRemainingContractualTerm" id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsUnvestedOutstandingWeightedAverageRemainingContractualTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedOutstandingIntrinsicValue" id="alt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsNonvestedOutstandingIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="MedicalTechnologyEnterpriseConsortiumCollaborativeArrangementMember" id="alt_MedicalTechnologyEnterpriseConsortiumCollaborativeArrangementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="BARDAContractMember" id="alt_BARDAContractMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="DurationOfContract" id="alt_DurationOfContract" type="xbrli:durationItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommitmentsAndContingenciesLineItems" id="alt_CommitmentsAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommitmentsAndContingenciesTable" id="alt_CommitmentsAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="LonzaManufacturingAgreementMember" id="alt_LonzaManufacturingAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CommitmentToProcureEquipmentAndConstruction" id="alt_CommitmentToProcureEquipmentAndConstruction" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" abstract="false" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="CommonStockIssuedRelatedToContingentConsiderationLiability" id="alt_CommonStockIssuedRelatedToContingentConsiderationLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="credit" xbrli:periodType="duration" />
  <xsd:element name="AdditionOperatingLeaseLiabilityAndRightOfUseAsset" id="alt_AdditionOperatingLeaseLiabilityAndRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="duration" />
  <xsd:element name="ClinicalTrialNotificationMember" id="alt_ClinicalTrialNotificationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SharesIssuedForContingentMilestonePayment" id="alt_SharesIssuedForContingentMilestonePayment" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="PeriodForSubmissionOfClinicalTrialNotification" id="alt_PeriodForSubmissionOfClinicalTrialNotification" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="SubmissionDateOfClinicalTrialNotification" id="alt_SubmissionDateOfClinicalTrialNotification" type="xbrli:dateItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="IncomeTaxesAndResearchDevelopmentIncentiveReceivableCurrent" id="alt_IncomeTaxesAndResearchDevelopmentIncentiveReceivableCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="IncreaseDecreaseInIncomeTaxesAndTaxIncentiveReceivable" id="alt_IncreaseDecreaseInIncomeTaxesAndTaxIncentiveReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="WarrantsExpired" id="alt_WarrantsExpired" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="PhaseTwoMilestoneMember" id="alt_PhaseTwoMilestoneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ExchangingStockholdersMember" id="alt_ExchangingStockholdersMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ContingentConsiderationPercentageOfSharesReleaseAfterThreeMonths" id="alt_ContingentConsiderationPercentageOfSharesReleaseAfterThreeMonths" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ContingentConsiderationPercentageOfSharesReleaseAfterSixMonths" id="alt_ContingentConsiderationPercentageOfSharesReleaseAfterSixMonths" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="DeferredOfferingCostsAppliedToProceeds" id="alt_DeferredOfferingCostsAppliedToProceeds" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="ResearchAndDevelopmentIncentiveCredit" id="alt_ResearchAndDevelopmentIncentiveCredit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ResearchAndDevelopmentIncentiveCreditEarned" id="alt_ResearchAndDevelopmentIncentiveCreditEarned" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
</xsd:schema>
