Altimmune, Inc. 10-K

Item 8. Financial Statements and Supplementary Data ​ ALTIMMUNE, INC. INDEX TO CONSOLIDATED FINANCIAL STATEMENTS ​ ​ Report of Independent Registered Public Accounting Firm — Ernst & Young LLP (PCAOB ID: 42 ) 90 Consolidated Balance Sheets as of December 31, 2025 and 2024 92 Consolidated Statements of Operations and Comprehensive Loss for the years ended December 31, 2025 and 2024 93 Consolidated Statements of Stockholders’ Equity for the years ended December 31, 2025 and 2024 94 Consolidated Statements of Cash Flows for the years ended December 31, 2025 and 2024 95 Notes to Consolidated Financial Statements 96 1. Nature of Business and Organization 96 2. Summary of Significant Accounting Policies 96 3. Fair Value Measurement 102 4. Property and Equipment, Net 104 5. Leases 104 6. Accrued Expenses and Other Current Liabilities 105 7. Term Loan 105 8. Other Noncurrent Liabilities 107 9. Stockholders’ Equity 107 10. Stock-Based Compensation 108 11. Employee Benefit Plans 111 12. Income Taxes 111 13. Net Loss Per Share 114 14. Commitments and Contingencies 114 15. Segment Information 115 16. Subsequent Events 115 ​ ​ ​ ​ 89 Table of Contents REPORT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM To the Shareholders and the Board of Directors of Altimmune, Inc. Opinion on the Financial Statements We have audited the accompanying consolidated balance sheets of Altimmune, Inc. (the Company) as of December 31, 2025 and 2024, the related consolidated statements of operations and comprehensive loss, changes in stockholders' equity and cash flows for each of the two years in the period ended December 31, 2025, and the related notes (collectively referred to as the “consolidated financial statements”). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company at December 31, 2025 and 2024, and the results of its operations and its cash flows for each of the two years in the period ended December 31, 2025, in conformity with U.S. generally accepted accounting principles. Basis for opinion These financial statements are the responsibility of the Company's management. Our responsibility is to express an opinion on the Company’s financial statements based on our audits. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) (PCAOB) and are required to be independent with respect to the Company in accordance with the U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB. We conducted our audits in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement, whether due to error or fraud. The Company is not required to have, nor were we engaged to perform, an audit of its internal control over financial reporting. As part of our audits we are required to obtain an understanding of internal control over financial reporting but not for the purpose of expressing an opinion on the effectiveness of the Company's internal control over financial reporting. Accordingly, we express no such opinion. Our audits included performing procedures to assess the risks of material misstatement of the financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the financial statements. Our audits also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audits provide a reasonable basis for our opinion. Critical Audit Matter The critical audit matter communicated below is a matter arising from the current period audit of the financial statements that was communicated or required to be communicated to the audit committee and that: (1) relates to accounts or disclosures that are material to the financial statements and (2) involved our especially challenging, subjective or complex judgments. The communication of the critical audit matter does not alter in any way our opinion on the consolidated financial statements, taken as a whole, and we are not, by communicating the critical audit matter below, providing a separate opinion on the critical audit matter or on the accounts or disclosures to which it relates. 90 Table of Contents ​ ​ ​ ​ ​ Accrued and Prepaid Research and Development Expenses Description of the Matter ​ As disclosed in Note 6, the Company’s total accrued research and development expenses were $6.1 million as of December 31, 2025, which represents the estimated obligation for research and development expenses incurred as of December 31, 2025 but not paid as of that date. In addition, the Company’s total prepaid expenses and other current assets were $3.0 million as of December 31, 2025, which included amounts that were paid in advance of services incurred pursuant to research and development activities. As discussed in Note 2 of the consolidated financial statements, material advance payments for goods or services that will be used or rendered for future research and development activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. When evaluating the adequacy of the accrued expenses, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. How We Addressed the Matter in Our Audit ​ Auditing the Company’s accrued and prepaid research and development expenses was especially challenging as the amounts are based on various estimates from third-party vendors, as well as other inputs estimated by members of management, such as actual costs incurred but not yet billed, estimated project timelines, and the costs associated with these services. Additionally, due to the timing of invoicing received from third parties, the actual amounts incurred are not always known by the report date. To evaluate the accrued and prepaid research and development expenses, our audit procedures included, testing the completeness and accuracy of the underlying data used to estimate the amounts recorded. We corroborated the progress of research and development activities through discussion with the Company’s research and development personnel that oversee the research and development projects. We inspected the Company’s contracts with third parties and any pending change orders to assess the impact on amounts recorded and inspected subsequent disbursements made and invoices received from third parties. Additionally, we independently confirmed research and development costs incurred and prepaid balances held as of December 31, 2025 with a sample of third parties. ​ /s/ Ernst & Young LLP We have served as the Company’s auditor since 2017. Iselin, New Jersey March 6, 2026 ​ 91 Table of Contents ALTIMMUNE, INC. CONSOLIDATED BALANCE SHEETS (In thousands, except share and per-share amounts) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ ​ 2025 ​ 2024 ASSETS ​ ​ ​ ​ Current assets: ​ ​ ​ ​ Cash and cash equivalents ​ $ 43,760 ​ $ 36,926 Restricted cash ​ 42 ​ 42 Total cash, cash equivalents and restricted cash ​ 43,802 ​ 36,968 Short-term investments ​ 229,696 ​ 94,965 Accounts and other receivables ​ 1,219 ​ 544 Income tax and R&D incentive receivables ​ 518 ​ 2,573 Prepaid expenses and other current assets ​ 2,957 ​ 2,204 Total current assets ​ 278,192 ​ 137,254 Property and equipment, net ​ 312 ​ 413 Other assets ​ 1,425 ​ 1,639 Total assets ​ $ 279,929 ​ $ 139,306 LIABILITIES AND STOCKHOLDERS’ EQUITY ​ ​ ​ ​ Current liabilities: ​ ​ ​ ​ Accounts payable ​ $ 2,717 ​ $ 211 Accrued expenses and other current liabilities ​ 12,280 ​ 10,257 Total current liabilities ​ 14,997 ​ 10,468 Term loan, noncurrent ​ ​ 34,287 ​ ​ — Other noncurrent liabilities ​ 5,753 ​ 5,330 Total liabilities ​ 55,037 ​ 15,798 Commitments and contingencies (Note 14) ​ ​ ​ ​ Stockholders’ equity: ​ ​ ​ ​ Common stock, $ 0.0001 par value; 200,000,000 shares authorized; 110,882,735 and 72,352,701 shares issued and outstanding as of December 31, 2025 and 2024, respectively ​ ​ 11 ​ ​ 7 Additional paid-in capital ​ 879,292 ​ 689,864 Accumulated deficit ​ ( 649,483 ) ​ ( 561,390 ) Accumulated other comprehensive loss, net ​ ( 4,928 ) ​ ( 4,973 ) Total stockholders’ equity ​ 224,892 ​ 123,508 Total liabilities and stockholders’ equity ​ $ 279,929 ​ $ 139,306 ​ The accompanying notes are an integral part of the consolidated financial statements. ​ 92 Table of Contents ALTIMMUNE, INC. CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (In thousands, except share and per-share amounts) ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2025 ​ 2024 Revenues ​ $ 41 ​ $ 20 Operating expenses: ​ ​ ​ ​ Research and development ​ 66,432 ​ 82,226 General and administrative ​ 28,098 ​ 20,966 Total operating expenses ​ 94,530 ​ 103,192 Loss from operations ​ ( 94,489 ) ​ ( 103,172 ) Other income (expense): ​ ​ ​ ​ Interest expense ​ ( 1,636 ) ​ ( 9 ) Interest income ​ 7,541 ​ 8,074 Other income (expense), net ​ ( 190 ) ​ 48 Total other income (expense), net ​ 5,715 ​ 8,113 Net loss before income taxes ​ ( 88,774 ) ​ ( 95,059 ) Income tax expense (benefit) ​ ( 681 ) ​ — Net loss ​ ( 88,093 ) ​ ( 95,059 ) Other comprehensive income — unrealized gain on short-term investments ​ 45 ​ 31 Comprehensive loss ​ $ ( 88,048 ) ​ $ ( 95,028 ) Net loss per share, basic and diluted ​ $ ( 1.00 ) ​ $ ( 1.34 ) Weighted-average common shares outstanding, basic and diluted ​ 88,104,132 ​ 71,003,399 ​ The accompanying notes are an integral part of the consolidated financial statements ​ ​ ​ ​ 93 Table of Contents ALTIMMUNE, INC. CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ EQUITY (In thousands, except share amounts) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Additional ​ ​ ​ ​ Other ​ Total ​ ​ ​ ​ Common Stock ​ ​ ​ Paid-In ​ ​ ​ Accumulated ​ ​ ​ Comprehensive ​ ​ ​ Stockholders’ ​ ​ Shares ​ Amount ​ Capital ​ Deficit ​ Loss ​ Equity Balance at December 31, 2023 ​ 70,677,400 $ 7 $ 665,427 $ ( 466,331 ) $ ( 5,004 ) ​ $ 194,099 Stock-based compensation — ​ — ​ 14,393 ​ — ​ — ​ 14,393 Exercise of stock options 138,386 ​ — ​ ​ 439 ​ — ​ — ​ 439 Vesting of restricted stock awards including withholding, net 192,537 ​ — ​ ​ ( 850 ) ​ — ​ — ​ ( 850 ) Issuance of common stock from Employee Stock Purchase Plan 86,857 ​ ​ — ​ ​ 300 ​ — ​ — ​ 300 Issuance of common stock in at-the-market offerings, net ​ 1,207,521 ​ ​ — ​ ​ 9,994 ​ — ​ — ​ ​ 9,994 Issuance of common stock upon cashless exercise of warrants ​ 50,000 ​ — ​ ​ 161 ​ ​ — ​ ​ — ​ ​ 161 Unrealized gain on short-term investments ​ — ​ — ​ — ​ — ​ 31 ​ ​ 31 Net loss — ​ — ​ — ​ ( 95,059 ) ​ — ​ ( 95,059 ) Balance at December 31, 2024 72,352,701 ​ 7 ​ 689,864 ​ ( 561,390 ) ​ ( 4,973 ) ​ 123,508 Stock-based compensation — ​ — ​ ​ 16,076 ​ ​ — ​ ​ — ​ 16,076 Exercise of stock options 14,167 ​ — ​ ​ 41 ​ ​ — ​ ​ — ​ 41 Vesting of restricted stock awards including withholding, net 180,958 ​ — ​ ​ ( 709 ) ​ ​ — ​ ​ — ​ ( 709 ) Issuance of common stock from Employee Stock Purchase Plan 71,342 ​ ​ — ​ ​ 291 ​ ​ — ​ ​ — ​ 291 Issuance of common stock in at-the-market offerings, net ​ 38,263,567 ​ ​ 4 ​ ​ 173,729 ​ ​ — ​ ​ — ​ ​ 173,733 Unrealized gain on short-term investments — ​ — ​ ​ — ​ ​ — ​ ​ 45 ​ 45 Net loss — ​ — ​ ​ — ​ ​ ( 88,093 ) ​ ​ — ​ ( 88,093 ) Balance at December 31, 2025 110,882,735 ​ $ 11 ​ $ 879,292 ​ $ ( 649,483 ) ​ $ ( 4,928 ) ​ $ 224,892 ​ The accompanying notes are an integral part of the consolidated financial statement ​ ​ ​ 94 Table of Contents ALTIMMUNE, INC. CONSOLIDATED STATEMENTS OF CASH FLOWS (In thousands) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ December 31, ​ ​ 2025 ​ 2024 CASH FLOWS FROM OPERATING ACTIVITIES: ​ ​ ​ ​ Net loss ​ $ ( 88,093 ) ​ $ ( 95,059 ) Adjustments to reconcile net loss to net cash used in operating activities: ​ ​ ​ ​ Stock-based compensation expense ​ 16,076 ​ 14,393 Depreciation of property and equipment ​ 112 ​ 238 Accretion of discounts on short-term investments ​ ​ ( 2,224 ) ​ ​ ( 3,850 ) Amortization of debt discount and costs ​ ​ 363 ​ ​ — Loss on foreign currency exchange ​ 160 ​ ( 48 ) Deferred income tax benefit ​ ​ ( 681 ) ​ ​ — Changes in operating assets and liabilities: ​ ​ ​ ​ Accounts receivable ​ ( 675 ) ​ 567 Prepaid expenses and other assets ​ ( 204 ) ​ 4,713 Accounts payable ​ 2,506 ​ ( 1,859 ) Accrued expenses and other liabilities ​ 2,389 ​ ( 112 ) Income tax and R&D incentive receivables ​ 2,736 ​ 1,169 Net cash used in operating activities ​ ( 67,535 ) ​ ( 79,848 ) CASH FLOWS FROM INVESTING ACTIVITIES: ​ ​ ​ ​ Proceeds from sales and maturities of short-term investments ​ 152,986 ​ 87,345 Purchases of short-term investments ​ ( 285,448 ) ​ ( 115,731 ) Purchases of property and equipment ​ ( 11 ) ​ — Net cash used in investing activities ​ ( 132,473 ) ​ ( 28,386 ) CASH FLOWS FROM FINANCING ACTIVITIES: ​ ​ ​ ​ Payments of deferred offering costs ​ ​ ( 438 ) ​ ​ — Proceeds from exercises of warrants ​ — ​ 161 Proceeds from term loan ​ ​ 35,000 ​ ​ — Payment for debt issuance costs ​ ​ ( 1,076 ) ​ ​ — Proceeds from issuance of common stock in at-the-market offerings, net ​ 173,733 ​ 9,994 Proceeds from issuance of common stock from Employee Stock Purchase Plan ​ 291 ​ 300 Proceeds from exercises of stock options ​ 41 ​ 439 Payments for tax withholding in share-based compensation ​ ( 709 ) ​ ( 850 ) Net cash provided by financing activities ​ 206,842 ​ 10,044 Net increase (decrease) in cash and cash equivalents and restricted cash ​ 6,834 ​ ( 98,190 ) Cash, cash equivalents and restricted cash at beginning of period ​ 36,968 ​ 135,158 Cash, cash equivalents and restricted cash at end of period ​ $ 43,802 ​ $ 36,968 ​ ​ ​ ​ ​ ​ ​ SUPPLEMENTAL CASH FLOW INFORMATION: ​ ​ ​ ​ Cash paid for interest ​ $ 975 ​ $ — Cash received from income tax refunds ​ $ 1,422 ​ $ — SUPPLEMENTAL NON-CASH ACTIVITIES: ​ ​ ​ ​ Deferred offering costs in accrued expenses and other current liabilities ​ $ 111 ​ $ — Operating lease liability and right-of-use asset addition ​ $ — ​ $ 1,409 ​ The accompanying notes are an integral part of the consolidated financial statements. ​ 95 Table of Contents ALTIMMUNE, INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS 1. Nature of Business and Organization Altimmune, Inc., headquartered in Gaithersburg, Maryland, United States, together with its subsidiaries (collectively, the “Company” or “Altimmune”) is a late clinical-stage biopharmaceutical company incorporated under the laws of the State of Delaware. The Company is developing novel therapies for serious liver diseases. The Company’s lead product candidate is pemvidutide (formerly known as ALT-801), a balanced 1:1 glucagon/GLP-1 dual receptor agonist in development for the treatment of MASH, AUD and ALD. The Company may also pursue additional indications for pemvidutide that leverage its differentiated clinical profile. Since its inception, the Company has devoted substantially all of its efforts to business planning, research and development, recruiting management and technical staff, and raising capital, and has financed its operations through the issuance of common and preferred stock, long-term debt, and proceeds from research grants and government contracts. The Company has not generated any revenues from the sale of any products to date, and there is no assurance of any future revenues from product sales. 2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are prepared in conformity with generally accepted accounting principles in the United States (“U.S. GAAP”) and in accordance with the rules and regulations of the United States Securities and Exchange Commission (“SEC”). The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All significant intercompany accounts and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to, the valuation of share-based awards, income taxes, prepaids and accruals for research and development (“R&D”) activities. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable. However, actual results could differ from those estimates. Segment Information The Company is being managed and operated as a single business developing novel therapies for serious liver diseases. The Company is managed by a single management team, and consistent with its organizational structure, the Chief Executive Officer manages and allocates resources at a consolidated level. Accordingly, the Company views its business as one operating segment. Cash Equivalents The Company considers all highly liquid investments purchased with remaining maturities of 90 days or less on the purchase date to be cash equivalents, and includes amounts held in money market funds and commercial paper which are actively traded (a Level 1 input). Restricted Cash The Company had restricted cash of $ 42,000 as of each of December 31, 2025 and 2024, held in money market savings accounts as collateral for its facility lease obligation. Restricted cash is classified as a component of cash, cash 96 Table of Contents equivalents, and restricted cash in the accompanying Consolidated Balance Sheets and Consolidated Statements of Cash Flows. Short-term Investments The Company’s short-term investments are comprised of U.S. Treasuries, corporate debt securities, certificates of deposit, asset backed securities and agency debt securities that have original maturities less than or equal to one year and are classified as available-for-sale (“AFS”) securities. Such securities are carried at estimated fair value, net of allowance for credit loss determined based on the Current Expected Credit Loss. Any unrealized holding gains or losses are reported as accumulated other comprehensive income or loss, which is a separate component of stockholders’ equity. In the event that the AFS security's fair value is below the amortized cost and (i) the Company intends to sell the AFS security and (ii) the AFS security is required to be sold before recovery of the loss, the AFS security's amortized cost base will be written down to its fair value and the loss will be recognized in the income statement. If the Company intends not to sell the AFS security and the AFS security is not required to be sold before recovery of the loss, the Company evaluates whether a portion of the unrealized loss is a result of credit loss. The portion of unrealized loss related to credit loss will be recorded as allowance for credit loss in the balance sheet with the corresponding credit loss in the income statement and the portion of unrealized loss not related to credit loss will be recognized in other comprehensive income (“OCI”). Dividend and interest income are recognized in other income when earned. The cost of securities sold is calculated using the specific identification method. The Company places all investments with government agencies, or corporate institutions whose debt is rated as investment grade. As of December 31, 2025 and 2024, none of the unrealized losses on the Company’s short-term investments are a result of credit loss, and therefore, any unrealized losses were recognized in OCI. As of December 31, 2025 and 2024, the Company had $ 1.1 million and $ 0.4 million, respectively, accrued interest on short-term investments included in “Accounts and other receivables” on the accompanying Consolidated Balance Sheets. Accounts and Other Receivables Accounts and other receivables include interest and other receivables. The Company makes judgments as to its ability to collect outstanding receivables and provides an allowance for receivables when collection becomes doubtful. Provisions are made based upon a specific review of all significant outstanding invoices and the overall quality and age of those invoices not specifically reviewed. The Company believes that the credit risks associated with these receivables are not significant. To date, the Company has not experienced any losses associated with accounts and other receivables and does not maintain an allowance for credit loss. Fair Value Measurements The Company records certain financial assets and liabilities at fair value in accordance with the guidance in Financial Accounting Standards Board (“FASB”) Accounting Standard Codification Topic 820, Fair Value Measurements and Disclosures (“ASC 820”), which defines fair value and establishes a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 — Quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company can access at the measurement date. Level 2 — Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term. Level 3 — Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use. 97 Table of Contents To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment that the Company exercises in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. There were no transfers into or out of Level 3 of the fair value hierarchy during the years ended December 31, 2025 and 2024. Financial Instruments The Company’s financial instruments consist of cash, cash equivalents, restricted cash, accounts receivable, short-term investments, accounts payable, accrued expenses, and a term loan. The carrying amounts of cash, cash equivalents, restricted cash, accounts and other receivables, accounts payables, and accrued expenses approximate their fair value due to the short-term nature of those financial instruments. Short-term investments are recorded at fair value, with any unrealized holding gains or losses reported as accumulated other comprehensive income or loss. The carrying amounts of the Company’s debt approximate fair value because the rates are floating rates based on the prime lending rate, which approximates market rates (see Note 7) and represents a Level 2 fair value measurement. Concentrations of Credit Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents, restricted cash, short-term investments and accounts receivable. Periodically, the Company maintains deposits in financial institutions in excess of government insured limits. Management believes that the Company is not exposed to significant credit risk as the Company’s deposits are held at financial institutions that management believes to be of high credit quality. The Company has not experienced any losses in these deposits. Property and Equipment, Net The Company records property and equipment at cost less accumulated depreciation and amortization. Expenditures for maintenance and repairs are charged to operations as incurred, whereas major improvements are capitalized as additions to property and equipment. Costs of assets under construction are capitalized but are not depreciated until the construction is substantially complete and the assets being constructed are ready for their intended use. Depreciation and amortization are recorded using the straight-line method over the estimated useful lives of the assets, as follows: ​ ​ ​ Asset Category ​ ​ ​ Estimated Useful Life Computer and telecommunications 3 – 5 years Software 3 years Furniture, fixtures and equipment 5 years Laboratory equipment 7 years Leasehold improvements Lesser of lease term or estimated useful lives ​ Impairment or Disposal of Long-lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate the carrying value of such assets may not be recoverable in accordance with the guidance in Financial Accounting Standards Board (“FASB”) Accounting Standard Codification Topic 360, Property, Plant and Equipment (“ASC 360”). The Company’s long-lived assets include properties and equipment and right of use (“ROU”) assets. For long-lived assets, impairment is recognized when the undiscounted cash flows used in the test for recoverability is less than their carrying value. In the event impairment exists, the long-lived asset will be written down to its fair value, and an impairment loss is recorded as the difference between the carrying value and fair value. For the years ended December 31, 2025 and 2024, the Company did no t identify indicators of impairment. 98 Table of Contents Leases The Company determines if an arrangement is a lease at inception. Operating leases are recorded as a current and long-term lease obligation, with a corresponding right of use lease assets. Lease liabilities represent the Company’s obligation to make lease payments arising from leases. ROU assets represent the Company’s right to use an underlying asset for the lease term. Lease liabilities and ROU assets are recognized at the lease commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at the lease commencement date in determining the present value of lease payments. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise those options. Lease expense for lease payments is recognized on a straight-line basis over the lease term. Short-term leases are leases having a term of twelve months or less. The Company recognizes short-term leases on a straight-line basis and does not record a related lease asset or liability for such leases. Lease incentives and allowance provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations as the related construction costs are incurred, up to the maximum allowance. Stock-based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. The Company estimates the fair value of stock options granted using the Black-Scholes option pricing model on the dates of grant. For restricted stock and restricted stock units granted, fair value is determined based on the grant date closing price of the Company’s common stock. The expected stock price volatility for stock option awards is based on a weighted-average volatility rate of historical volatility from a representative peer group of comparable companies and the Company’s own stock price volatility. The average expected life of stock options was determined according to the “simplified method” as described in SAB 107, which is the midpoint between the vesting date and the end of the contractual term. The risk-free interest rate was determined by reference to implied yields available from U.S. Treasury securities with a remaining term equal to the expected life assumed at the date of grant. We have not paid and do not anticipate paying cash dividends. Therefore, the expected dividend rate is assumed to be 0%. Stock-based compensation awarded to employees is measured at the grant date fair value of stock option grants and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. The Company estimates forfeitures at the time of grant and, if necessary, revises the estimate in subsequent periods if actual forfeitures differ from those estimates. Estimates are based on the Company’s historical analysis of actual stock option forfeitures. The actual expense recognized over the vesting period is only for those options that vest. If awards are modified, the Company compares the fair value of the affected award measured immediately prior to modification to its value after modification. To the extent that the fair value of the modified award exceeds the original award, the incremental fair value of the modified award is recognized as compensation expense on the date of modification for vested awards, and over the remaining vesting period for unvested awards. R&D Expense R&D costs are expensed as incurred. R&D costs consist of payroll and personnel expense, consulting costs, external contract R&D expenses, which includes fees paid to other entities that conduct certain R&D activities on the Company’s behalf, such as clinical research organizations (“CROs”) and contract manufacturing organizations (“CMOs”), raw materials, drug product manufacturing costs, laboratory supplies and allocated overhead, including depreciation and amortization, rent and utilities. Material R&D costs that are paid in advance of performance are capitalized as a prepaid expense and amortized over the service period as the services are provided. 99 Table of Contents Clinical trial costs are a significant component of R&D expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include investigator fees, site and patient costs, CRO costs, costs for central laboratory testing, data management and CMO costs. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations and other pass-through costs. These inputs are required to be estimated due to a lag in receiving the actual clinical information from third parties. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the Consolidated Balance Sheets as a prepaid asset or accrued expenses. These third-party agreements are generally cancelable, and related costs are recorded as R&D expenses as incurred. Material advance payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. When evaluating the adequacy of the accrued expenses, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the prepaid and accrued balances at the end of any reporting period. R&D Incentive Credits The Company is eligible to obtain certain R&D incentive credits, through the participation in the U.K. Merged R&D Expenditure Credit (“U.K. RDEC”) and the Australian R&D Tax Incentive (the “Australia R&D credit”) program administered through the Australian Tax Office (the “ATO”). The U.K. RDEC are calculated as a percentage of qualifying R&D expenses and are payable in cash by the U.K. government to the Company. Qualifying R&D expenses consist of employment costs for research staff, consumables, a proportion of relevant, permitted sub-contract costs and certain internal overhead costs incurred as part of research projects for which the Company does not receive income. The Australia R&D credits provide for a cash refund based on a percentage of certain R&D activities undertaken in Australia by the Company’s wholly owned subsidiary, Altimmune AU Pty, Limited. Qualifying R&D expenses must be incurred within the country. The U.K. and Australian incentive credits are available on the basis of specific criteria with which the Company must comply. The incentive credits are subject to future audits by the government authorities and a statute of limitations. Although the incentive credits may be administered through the local tax authority, the Company has accounted for the incentives outside of the scope of FASB Accounting Standards Codification Topic 740, Income Taxes (“ASC 740”), since the incentives are not linked to the Company’s taxable income and can be realized regardless of whether the Company has generated taxable income in the respective jurisdictions. The Company accounts for these incentive credits as a government grant which analogizes with International Accounting Standards 20 (“IAS 20”), Accounting for Government Grants and Disclosure of Government Assistance . The Company records qualifying U.K. R&D expenses as receivable and a corresponding reduction to R&D expense in the Consolidated Statement of Operations and Comprehensive Loss. During the years ended December 31, 2025 and 2024, the Company recognized a negligible amount and $ 0.6 million, respectively, of R&D credits as a reduction to R&D expense in the Consolidated Statement of Operations and Comprehensive Loss. As of December 31, 2025 and 2024, the Company had $ 0.5 million and $ 1.8 million, respectively, of RDEC included in “Income tax and R&D incentive receivables” on the accompanying Consolidated Balance Sheets. The Company records qualifying Australian R&D credits as receivable with a full valuation reserve. Cash receipts for Australia R&D credits are recorded as noncurrent liability until it either passes an audit performed by the ATO, or the statute of limitations ends, whichever occurs first. Upon successfully passing an audit or the expiration of the statute of limitations, the Company will clear the liability and a corresponding reduction to R&D expense unless recognition criteria is met in a later year, in which case the R&D credit will be recorded as other income in the Consolidated Statement of Operations and Comprehensive Loss. During the years ended December 31, 2025 and 2024, the Company received $ 0.4 million and $ 0.1 million in cash for R&D incentive, respectively, related to the R&D costs that the Company incurred during the fiscal years 2024 and 2023, respectively, both through the participation in the Australian R&D credit program, and is included in “Other noncurrent liabilities” on the accompanying Consolidated Balance Sheets. 100 Table of Contents Income Taxes The Company accounts for income taxes in accordance with ASC 740. ASC 740 uses the asset and liability approach, which requires the recognition of future tax benefits or liabilities on the temporary differences between the financial reporting and tax bases of our assets and liabilities. Deferred tax assets and liabilities represent future tax consequences of temporary differences between the financial statement carrying amounts and the tax basis of assets and liabilities and for loss carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized. The Company also recognizes a tax benefit from uncertain tax positions only if it is “more likely than not” that the position is sustainable based on its technical merits. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Should such costs be incurred, they would be classified as a component of provision for income taxes. The Company conducts R&D activities potentially qualified to claim research tax credits for U.S. federal and state purposes under Internal Revenue Code Section 41. The Company has not performed a formal study claiming these credits in the tax returns because the Company does not yet have taxable profits. Once the Company becomes profitable, it will likely have a study prepared, and the amount of R&D tax credits available could generate income tax benefit, subject to an annual Section 383 limitation and valuation allowance for realizability of the deferred tax asset. Comprehensive Loss For the years presented, the total comprehensive loss includes net loss and other comprehensive income (loss) which represents unrealized gains or losses on short-term investments. Net Loss per Share Basic net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period without consideration for potentially dilutive securities. The Company computes diluted net loss per common share after giving consideration to all potentially dilutive common equivalents, including all unvested restricted stock and stock options outstanding during the period except where the effect of such non-participating securities would be anti-dilutive. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and dilutive common stock equivalents outstanding for the period determined using the treasury-stock and if-converted methods. Debt discount and issuance costs The Company accounts for fees paid to lenders, as compensation for services beyond their role as a creditor, and third parties whose costs are directly related to issuing debt as debt issuance cost. Amounts paid to the lender as a reduction in the proceeds received are considered a discount on the issuance. Debt issuance costs and discounts related to term loans are reported as a direct deduction from the outstanding debt and amortized over the term of the loan using the effective interest method as an additional interest expense. Recently issued accounting pronouncements Recently Adopted: In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures . ASU No. 2023-09 requires public business entities on an annual basis to (i) disclose in the rate reconciliation both percentages and amounts for certain categories in a tabular format, with further disaggregation of certain categories when the individual reconciling items meet a quantitative threshold, (ii) disclose income taxes paid, net of refunds received disaggregated by federal, state and foreign, with further disaggregation by individual jurisdictions that meet a qualitative threshold (iii) eliminates the requirement to disclose certain information when it is reasonably possible that the total 101 Table of Contents amounts of unrecognized tax benefits will significantly increase or decrease within 12 months of the reporting date or make a statement that an estimate of the range cannot be made and (iv) eliminates the requirement to disclose cumulative amount of each type of temporary difference in certain circumstances. The amendments in ASU 2023-09 are effective for fiscal years beginning after December 15, 2024, with early adoption permitted for annual financial statements that have not yet been issued. The Company adopted ASU No. 2023-09 on this annual report on Form 10-K using a prospective method (See Note 12. Income Taxes). Not Yet Adopted: In December 2025, the FASB issued ASU No. 2025-10, Government Grants – Accounting for Government Grants Received by Business Entities (Topic 832) . The amendments in ASU No. 2025-10 establish the accounting for a government grant received by a business entity, including guidance for (1) a grant related to an asset and (2) a grant related to income. A grant related to an asset is a government grant, or part of a government grant, that is conditioned on the purchase, construction, or acquisition of an asset (for example, a long-lived asset or inventory). A grant related to income is a government grant, or part of a government grant, other than a grant related to an asset (for example, a grant that reimburses a business entity for operating expenses). The amendment require that a government grant received by a business entity should not be recognized until (1) It is probable that (a) a business entity will comply with the conditions attached to the grant and (b) the grant will be received and (2) business entity meets the recognition guidance for a grant related to an asset or a grant related to income. The amendments in ASU 2025-10 are effective for annual reporting periods beginning after December 15, 2028. Early adoption is permitted. The Company is currently evaluating the impact of this amendment on its Consolidated Financial Statements. In November 2024, the FASB issued ASU No. 2024-03, Income Statement – Reporting Comprehensive Income-Expense Disaggregation Disclosure (Subtopic 220-40) . The amendments in ASU No. 2024-03 require that, at each interim and annual reporting period, public business entities disclose in a note to the financial statements the amount of (i) purchases of inventory, (ii) employee compensation, (iii) depreciation, (iv) intangible asset amortization, and (v) depreciation, depletion, and amortization recognized as part of oil and gas-producing activities (DD&A) (or other amounts of depletion expense) included in each relevant expense caption in the statements of operation and comprehensive income. The amendments in ASU 2024-03 are effective for annual reporting periods beginning after December 15, 2026, and interim reporting periods within annual reporting periods beginning after December 15, 2027. Early adoption is permitted. The amendments should be applied either prospectively to financial statements issued for reporting periods after the effective date or retrospectively to any or all prior periods presented in the financial statements. The amendments in this update are effective on the Company’s 2027 annual report, and interim reports thereafter. The Company is currently evaluating the impact of this amendment on its Consolidated Financial Statements. 3. Fair Value Measurement The Company records cash equivalents and short-term investments at fair value on a recurring basis. Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants based on assumptions that market participants would use in pricing an asset or liability. The Company’s assets measured at fair value on a recurring basis as of December 31, 2025 consisted of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurement at December 31, 2025 ​ ​ ​ ​ Total ​ ​ ​ Level 1 ​ ​ ​ Level 2 ​ ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash equivalents - money market funds ​ $ 23,780 ​ $ 23,780 ​ $ — ​ $ — Cash equivalents - commercial paper ​ ​ 5,969 ​ ​ — ​ ​ 5,969 ​ ​ — Short-term investments ​ 229,696 ​ — ​ 229,696 ​ — Total ​ $ 259,445 ​ $ 23,780 ​ $ 235,665 ​ $ — ​ 102 Table of Contents The Company’s assets measured at fair value on a recurring basis as of December 31, 2024 consisted of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurement at December 31, 2024 ​ ​ ​ ​ Total ​ ​ ​ Level 1 ​ ​ ​ Level 2 ​ ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash equivalents - money market funds ​ $ 27,279 ​ $ 27,279 ​ $ — ​ $ — Short-term investments ​ 94,965 ​ — ​ 94,965 ​ — Total ​ $ 122,244 ​ $ 27,279 ​ $ 94,965 ​ $ — ​ Short-term investments have been initially valued at the transaction price and subsequently valued at the end of each reporting period utilizing third party pricing services or other market observable data (Level 2). The pricing services utilize industry standard valuation models, including both income and market-based approaches and observable market inputs to determine value. Short-term investments with quoted prices as of December 31, 2025 as shown below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2025 ​ ​ ​ ​ Unrealized Gain ​ Unrealized Gain ​ ​ ​ ​ Amortized Cost ​ Unrealized (Loss) Gain ​ Credit loss ​ Market Value United States treasury securities ​ ​ ​ $ 83,730 ​ ​ ​ $ 62 ​ ​ ​ $ — ​ ​ ​ $ 83,792 Commercial paper and corporate debt securities ​ ​ 127,934 ​ ​ 41 ​ ​ — ​ ​ 127,975 Asset backed securities ​ 8,000 ​ 7 ​ — ​ 8,007 Agency debt securities ​ ​ 9,920 ​ ​ 2 ​ ​ — ​ ​ 9,922 Total ​ $ 229,584 ​ $ 112 ​ $ — ​ $ 229,696 ​ Short-term investments with quoted prices as of December 31, 2024 as shown below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2024 ​ ​ ​ ​ Unrealized Gain ​ Unrealized Gain ​ ​ ​ ​ ​ Amortized Cost ​ Unrealized (Loss) Gain ​ Credit Loss ​ Market Value United States treasury securities ​ ​ ​ $ 21,375 ​ ​ ​ $ 16 ​ ​ ​ $ — ​ ​ ​ $ 21,391 Commercial paper and corporate debt securities ​ ​ 52,641 ​ ​ 15 ​ ​ — ​ ​ 52,656 Asset backed securities ​ 5,951 ​ 14 ​ — ​ 5,965 Agency debt securities ​ ​ 14,931 ​ ​ 22 ​ ​ — ​ ​ 14,953 Total ​ $ 94,898 ​ $ 67 ​ $ — ​ $ 94,965 ​ The carrying amounts of the Company’s debt approximate fair value because the rates are floating rates based on the prime lending rate, which approximates market rates (see Note 7) and represents a Level 2 fair value measurement. Separate disclosure is required for assets and liabilities measured at fair value on a recurring basis from those measured at fair value on a non-recurring basis. Assets recorded at fair value on a non-recurring basis, such as property and equipment and intangible assets are recognized at fair value when they are impaired. During the years ended December 31, 2025 and 2024, the Company had no assets or liabilities that were measured at fair value on a non-recurring basis. 103 Table of Contents 4. Property and Equipment, Net Property and equipment, net consists of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ 2025 ​ 2024 Furniture, fixtures and equipment ​ ​ ​ $ 164 ​ 163 Laboratory equipment ​ 342 ​ 342 Computers and telecommunications ​ 204 ​ 194 Software ​ 178 ​ 178 Leasehold improvements ​ 1,749 ​ 1,749 Property and equipment, at cost ​ 2,637 ​ 2,626 Less: accumulated depreciation and amortization ​ ​ ( 2,325 ) ​ ​ ( 2,213 ) Property and equipment, net ​ $ 312 ​ $ 413 ​ Depreciation expense related to property and equipment for the years ended December 31, 2025 and 2024 was $ 0.1 million and $ 0.2 million, respectively. 5. Leases The Company’s operating lease consists of a lease for office and laboratory space in the United States, which expires in April 2030. The lease provides for increases in future minimum annual rental payments as defined in the lease agreement and has no further renewal option. Short-term leases are leases having a term of twelve months or less. The Company recognizes short-term leases on a straight-line basis and does not record a related lease asset or liability for such leases. Rent expense under the Company’s operating leases during the years ended December 31, 2025 and 2024 was $ 0.6 million and $ 0.5 million, respectively. Rent expense includes short-term leases and variable lease costs that are not included in the lease obligation. The cash paid for operating lease liabilities for the years ended December 31, 2025 and 2024 was $ 0.4 million and $ 0.5 million, respectively. Supplemental balance sheet information related to the operating leases is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2025 ​ 2024 ​ Balance Sheet Classification Operating lease obligations, current (see Note 6) ​ ​ ​ $ 256 ​ ​ ​ $ 279 ​ ​ Accrued expenses and other current liabilities Operating lease obligations, noncurrent (see Note 8) ​ ​ 1,145 ​ ​ 1,402 ​ ​ Other noncurrent liabilities Operating lease right-of-use assets ​ $ 1,425 ​ $ 1,639 ​ ​ Other assets Weighted-average remaining lease term (years) ​ 4.3 ​ 5.3 ​ ​ ​ Weighted-average discount rate ​ 9.5 % 9.5 % ​ ​ ​ Maturities of operating lease liabilities are as follows (in thousands): ​ ​ ​ ​ 2026 ​ $ 376 2027 ​ 387 2028 ​ 398 2029 ​ ​ 410 2030 ​ ​ 138 Total operating lease payments ​ 1,709 Less: imputed interest ​ ( 308 ) Total operating lease liabilities ​ $ 1,401 ​ ​ 104 Table of Contents 6. Accrued Expenses and Other Current Liabilities Accrued expense and other current liabilities consist of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2025 ​ 2024 Accrued professional services $ 923 ​ ​ ​ $ 401 Accrued payroll and employee benefits 4,676 ​ 3,079 Accrued research and development 6,110 ​ 6,443 Lease obligation, current portion 256 ​ 279 Accrued interest and other 315 ​ 55 Total accrued expenses and other current liabilities $ 12,280 ​ $ 10,257 ​ ​ ​ 7. Term Loan On May 13, 2025 (“Closing Date”), the Company and certain of its subsidiaries entered into a Loan and Security Agreement (“Loan Agreement”) with Hercules Capital, Inc. (“Hercules”) and the lenders party thereto, pursuant to which the lenders will make available up to four tranches of term loans in an aggregate principal amount of $ 100.0 million (the “Term Loan”), subject to certain terms and conditions. Under the terms of the Loan Agreement, the first Term Loan tranche was drawn down on the Closing Date in an aggregate principal amount of $ 15.0 million. Upon the achievement of certain milestones and subject to other terms and conditions set out in the Loan Agreement, (i) the second Term Loan tranche was to be made available in an aggregate principal amount of up to $ 25.0 million and (ii) the third Term Loan tranche was to be made available in an aggregate principal amount of up to $ 15.0 million. The fourth Term Loan tranche was to be made available in an aggregate principal amount of up to $ 45.0 million subject to the approval of the lenders. The Term Loan had interest rate equal to the greater of (a) the prime rate as reported in The Wall Street Journal plus 2.45 % and (b) (i) 9.95 % until December 31, 2025, and (ii) 9.45 % thereafter. On November 5, 2025, (the “Amendment Closing”), the Company entered into an amendment to the Loan Agreement with Hercules and the lenders party thereto, pursuant to which the lenders will, subject to certain terms and conditions, increase the availability under the Term Loan from an aggregate principal amount of $ 100.0 million to $ 125.0 million. The Term Loan, as amended, is structured in four tranches. As described above, the first Term Loan tranche was drawn down on the Closing Date in an aggregate principal amount of $ 15.0 million. The second Term Loan tranche was drawn down on the Amendment Closing in an aggregate principal amount of $ 20.0 million. Upon the achievement of certain milestones and subject to other terms and conditions set out in the Loan Agreement, as amended, the third Term Loan tranche will be made available in an aggregate principal amount of up to $ 10.0 million. The fourth Term Loan tranche will be made available in an aggregate principal amount of up to $ 80.0 million subject to the approval of the lenders. The Term Loan, as amended, bears interest equal to the greater of (a) 9.70 % per annum and (b) the prime rate as reported in The Wall Street Journal plus 2.45 % per annum. The interest-only period has been extended to 30 months from May 13, 2025. The Term Loan will mature on January 1, 2029 (the “Maturity Date”). The Company may make payments of interest only through December 1, 2027, which will be extended to June 1, 2028, or December 1, 2028, if certain conditions described in the Loan Agreement, as amended, are met. Thereafter, the Company is obligated to make payments that will include installments of principal and interest through the Maturity Date. The Loan Agreement includes customary representations and warranties and covenants associated with the Term Loan. Such terms include (1) covenants concerning financial and other reporting obligations, and (2) certain limitations on indebtedness, liens, investments, distributions (including dividends), collateral, transfers, mergers or acquisitions, taxes, corporate changes, and deposit accounts. Compliance with the financial covenant will be conditionally waived pursuant to the terms of the Loan Agreement when the Company’s market capitalization exceeds $ 800 million. The Loan Agreement includes customary events of default, including payment defaults, breaches of representations and warranties, breaches of covenants following any applicable cure period and the occurrence of certain events that could reasonably be expected to have a “material adverse effect” as set forth in the Loan Agreement. As of December 31, 2025, the Company was in compliance with the covenants under the Loan Agreement. 105 Table of Contents The obligation under the Loan Agreement is secured by a security interest in substantially all of the Company’s assets and the assets of its subsidiaries that are co-borrowers or guarantors. Upon the occurrence of an event of default, Hercules will be entitled to exercise remedies, including acceleration of the Term Loan obligations and foreclosure on collateral. The Loan Agreement, as amended, provides for a prepayment charge equal to 3.0 % of the outstanding principal balance of the Term Loan if prepayment is made within the twelve months after Amendment Closing, 2.0 % if within the twenty-four months after Amendment Closing, 1.0 % if within the thirty-six months after Amendment Closing and 0.0 % thereafter. The Loan Agreement provides for an end of term charge of 6.25 % of the funded loan amount, due at the earlier of prepayment or maturity. Pro-rata payment of any earned end of term charge will be due upon any partial prepayment (the “End of Term Charge”). In addition, the Loan Agreement requires the Company to pay a facility charge of 1.0 % of the Term Loan funded due at the Closing Date and of each subsequent Term Loan tranche at the time such tranche is funded. The Company accounted for the End of Term Charge, Facility Charge, and other direct costs incurred in connection with the Loan Agreement and its amendment as a debt discount and issuance costs, and they are being amortized over the term of the loan using the effective interest method. The weighted-average effective interest rate on the Term Loan is 13.37 %. The Company incurred interest expense on the Term Loan, including debt discount and issuance costs amortization, of $ 1.6 million during the year ended December 31, 2025. The Term Loan consists of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ ​ 2025 ​ 2024 Term loan principal amount ​ ​ ​ $ 35,000 ​ ​ ​ $ — End of term charge ​ ​ 2,188 ​ ​ — Unamortized discount and issuance costs ​ ​ ( 2,901 ) ​ ​ — Total term loan ​ ​ 34,287 ​ ​ — Less: current portion of term loan ​ — ​ — Total term loan, net of current portion ​ $ 34,287 ​ $ — ​ Future principal loan payments on the currently outstanding Term Loan as of December 31, 2025 are as follows (in thousands): ​ ​ ​ ​ 2026 ​ $ — 2027 ​ 1,712 2028 ​ 21,611 2029 ​ ​ 11,677 Total future principal payments ​ 35,000 Add: End of term charge ​ ​ 2,188 Less: Unamortized discount and issuance costs ​ ( 2,901 ) Less: Current portion of term loan ​ ​ — Total term loan, net of current portion ​ $ 34,287 ​ ​ 106 Table of Contents 8. Other Noncurrent Liabilities The Company’s other noncurrent liabilities are summarized as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ 2025 ​ 2024 Research and development incentive credit $ 4,448 ​ $ 3,746 Lease obligation, long-term portion ​ 1,145 ​ ​ ​ ​ 1,402 Conditional economic incentive grants 160 ​ 160 Other — ​ 22 Total other noncurrent liabilities $ 5,753 ​ $ 5,330 ​ R&D Tax Incentive Program During the years ended December 31, 2025 and 2024, the Company received a total of $ 0.4 million and $ 0.1 million in cash for R&D incentive credit, respectively, related to R&D costs that the Company incurred during the fiscal year 2024 and 2023, respectively, through the participation in the Australian R&D credit program administered through the ATO. The Company recorded the receipt as noncurrent liability until there is reasonable assurance that the Company complies with the conditions attached to the incentive credit. Economic Incentive Grants The Company had two conditional economic incentive grants for a total of $ 250,000 from Montgomery County, Maryland and the State of Maryland. The Montgomery County grant of $ 100,000 was received in May 2018, with a term expiring on February 28, 2028 . The State of Maryland grant of $ 150,000 was received in October 2019, with a 10 -year term expiring on December 31, 2029 . These grants are conditional primarily based on the Company maintaining its current headquarter locations in addition to employing a required number of employees at different reporting dates through the term of the grants. The annual interest rate on both economic incentive grants is 3 %. During the year ended December 31, 2023, the Company repaid approximately $ 99,000 of the State of Maryland grant, including accrued interest, as the Company didn’t meet the required number of employees as of December 31, 2022 as per terms of the grant. 9. Stockholders’ Equity The Amended and Restated Certificate of Incorporation, as amended (“Charter”), authorized the Company to issue 200,000,000 shares of common stock, par value $ 0.0001 per share. As of December 31, 2025, the Company had 110,882,735 shares of common stock issued and outstanding . Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. The Charter also authorized the Company to issue 1,000,000 shares of preferred stock, par value $ 0.0001 per share. As of December 31, 2025, the Company had no shares of preferred stock issued and outstanding. At-the-Market Offerings On November 6, 2025, the Company entered into an Equity Distribution Agreement (the “November 2025 Agreement”) with Leerink Partners LLC serving as sales agent (the “November 2025 Sales Agent”) with respect to an at-the-market offerings program under which the Company may offer and sell, from time to time at its sole discretion, shares of its common stock, having an aggregate offering price of up to $ 200.0 million (the “November 2025 Shares”) through the November 2025 Sales Agent (the “November 2025 Offering”). All Shares offered and sold in the November 2025 Offering will be issued pursuant to the Company’s Registration Statement on Form S-3 filed with the SEC on February 27, 2025, which was declared effective on March 13, 2025, and the prospectus supplements related to the November 2025 Offering that form a part of the Registration Statement. The Company capitalized approximately $ 0.2 million of other offering costs which will offset the proceeds received from the shares sold under the November 2025 Agreement. During the year ended December 31, 2025, the Company has sold 5,899,059 shares of common stock under the November 2025 Agreement resulting in approximately $ 25.4 million in proceeds, net of $ 0.4 million commission and other offering costs, and as of December 31, 2025, $ 174.2 million remained available to be sold under the November 2025 Agreement. As of 107 Table of Contents December 31, 2025, there was $ 0.2 million deferred offering costs included in prepaid expenses and other current assets on the accompanying consolidated balance sheets. On February 27, 2025, the Company entered into an Equity Distribution Agreement (the “February 2025 Agreement”) with Leerink Partners LLC, Piper Sandler & Co. and Stifel, Nicolaus & Company, Incorporated, serving as sales agents (the “February 2025 Sales Agents”) with respect to an at-the-market offerings program under which the Company offered and sold, shares of its common stock, having an aggregate offering price of up to $ 150.0 million (the “February 2025 Shares”) through the February 2025 Sales Agents (the “February 2025 Offering”). All Shares offered and sold in the 2025 Offering were issued pursuant to the Company’s Registration Statement on Form S-3 filed with the SEC on February 27, 2025, which was declared effective on March 13, 2025, and the prospectus supplements related to the February 2025 Offering that form a part of the Registration Statement. During the year ended December 31, 2025, the Company has sold 27,896,642 shares of common stock under the February 2025 Agreement resulting in approximately $ 118.1 million in proceeds, net of $ 3.9 million commission and other offering costs. The February 2025 Agreement was terminated upon the execution of the November 2025 Agreement disclosed above. On February 28, 2023, the Company entered into an Equity Distribution Agreement (the “2023 Agreement”) with Evercore Group L.L.C., JMP Securities LLC and B. Riley Securities, Inc., serving as sales agents (the “2023 Sales Agents”), with respect to an at-the-market offerings program under which the Company offered and sold shares of its common stock, having an aggregate offering price of up to $ 150.0 million (the “2023 Shares”) through the Sales Agents (the “2023 Offering”). All Shares offered and sold in the 2023 Offering were issued pursuant to the Company’s Registration Statement on Form S-3 filed with the SEC on February 28, 2023, which was declared effective immediately, and the prospectus supplements relating to the 2023 Offering filed with the SEC on February 28, 2023. Since inception through the termination of the 2023 Agreement in February 2025, the Company has sold 26,129,903 shares of common stock under the 2023 Agreement resulting in approximately $ 126.8 million in proceeds, net of $ 4.1 million commission and other offering costs. During the years ended December 31, 2025 and 2024, we have sold 4,467,866 and 1,207,521 shares of common stock under the 2023 Agreement, resulting in approximately $ 30.2 million and $ 10.3 million in net proceeds, respectively. ​ ​ 10. Stock-Based Compensation Stock-based Compensation Plans The Company established the 2017 Omnibus Incentive Plan (the “Omnibus Plan”) to provide incentive stock options, non-qualified stock options, restricted stock, and other stock-based awards denominated in shares of the Company’s common stock, and performance-based cash awards to eligible employees, consultants and directors. In 2018, the Company’s shareholders approved an amendment to the Omnibus Plan to increase the number of shares reserved for issuance from 1,500,000 to 5,000,000 . The aggregate share reserve will be increased on January 1 of each year commencing in 2019 and ending on and including January 1, 2027 up to an amount equal to the lowest of (i) 4 % of the total number of shares of common stock outstanding on a fully diluted basis as of December 31 of the immediately preceding calendar year, and (ii) such number of shares of common stock, if any, determined by the Company’s board of directors. Accordingly, on January 1, 2026, the number of shares of Common Stock reserved and available for issuance under the Omnibus Plan increased by 4,969,458 . The maximum number of shares of common stock that may be issued under the Omnibus Plan in respect of Incentive Stock Option (“ISO”) is 5,000,000 shares. The maximum number of shares of common stock that may be granted to non-employee directors under the Omnibus Plan during any fiscal year is 2,000,000 shares. On November 29, 2018, the Board approved and adopted the Altimmune Inc. 2018 Inducement Grant Plan (the “Inducement Plan”). The Inducement Plan provides for the grant of equity or equity-based awards in the form of non-qualified stock options, restricted stock awards and other stock-based awards. The Inducement Plan was adopted by the Board without stockholder approval pursuant to Rule 5635I(4) of the Nasdaq Listing Rules. The Board has reserved 2,000,000 shares of the Company’s common stock for issuance pursuant to awards granted under the Inducement Plan (subject to customary adjustments in the event of a change in capital structure of the Company), and the Inducement Plan will be administered by the Compensation Committee. In accordance with 108 Table of Contents Rule 5635I(4) of the Nasdaq Listing Rules, awards under the Inducement Plan may be only made to an employee who has not previously been an employee or member of the Board or any parent or subsidiary, or following a bona fide period of non-employment by the Company or a parent or subsidiary, if he or she is granted such award in connection with his or her commencement of employment with the Company or a subsidiary and such grant is an inducement material to his or her entering into employment with the Company or such subsidiary. The Omnibus Plan and the Inducement Plan are collectively referred to as the “Plans.” Under both Plans, the shares of common stock underlying any awards that are expired, terminated, forfeited or canceled for any reason without having been exercised in full, except shares surrendered to satisfy tax withholding obligations, will be added to the shares of common stock available under each of the Plans. During the year ended December 31, 2025 under the Plans, a total of 5,468,350 options to purchase shares of common stock were granted. As of December 31, 2025, there were 316,510 and 409,575 shares of common stock available for future grants under the Omnibus Plan and the Inducement Plan, respectively. Stock Options The fair value of stock option issued to employees was estimated at the date of grant using Black-Scholes with the following weighted-average assumptions: ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ ​ ​ 2025 ​ ​ ​ 2024 Expected volatility 98.3 % 101.4 % Expected term (years) 6.0 6.0 ​ Risk-free interest rate 4.0 % 4.0 % Expected dividend yield 0.0 % 0.0 % ​ A summary of stock option activity under the Plans is presented below (in thousands, except share and per share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted-Average ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ Remaining ​ ​ ​ ​ ​ Number of ​ Average ​ Contractual Term ​ Aggregate Intrinsic ​ ​ Stock Options ​ Exercise Price ​ (Years) ​ Value Outstanding, December 31, 2024 6,630,477 ​ $ 9.32 6.0 ​ $ 5,839 Granted 5,468,350 ​ $ 5.21 ​ ​ ​ Exercised ( 14,167 ) ​ $ 2.91 ​ ​ ​ Forfeited or expired ( 794,740 ) ​ $ 9.34 ​ ​ ​ Outstanding, December 31, 2025 11,289,920 ​ $ 7.33 5.6 ​ $ 877 Exercisable, December 31, 2025 4,828,718 ​ $ 9.08 5.9 ​ $ 876 Vested and expected to vest, December 31, 2025 9,850,832 ​ $ 6.72 5.6 ​ $ 877 ​ The per share weighted-average grant date fair value of stock options granted during the years ended December 31, 2025 and 2024 were $ 4.16 and $ 7.07 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2025 and 2024 was $ 0.1 million and $ 1.1 million, respectively. The total fair value of options vested during the years ended December 31, 2025 and 2024 was $ 11.7 million and $ 11.6 million, respectively. As of December 31, 2025, there was $ 19.7 million of unrecognized compensation cost related to stock options, which is expected to be recognized over a weighted-average period of 3.1 years. Restricted Stock Units (RSUs) During the year ended December 2025, the Company granted 1,524,300 shares of RSUs with a weighted-average grant date fair value of $ 5.25 per share which vest over four years . During the year ended December 31, 2025, the Company issued 180,958 shares of common stock as a result of the vesting of 286,653 RSUs net of 105,695 shares of common stock withheld to satisfy tax withholding obligations. The total fair value of RSUs vested during the years ended December 31, 2025 and 2024 was $ 1.8 million and $ 2.5 million, respectively. 109 Table of Contents A summary of RSUs activities is presented below: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ average ​ ​ ​ ​ Grant Date ​ ​ Shares ​ Fair Value Unvested, December 31, 2024 858,290 ​ $ 10.12 Granted 1,524,300 ​ 5.25 Vested ( 286,653 ) ​ 10.60 Forfeited or expired ​ ( 32,137 ) ​ ​ 8.00 Unvested, December 31, 2025 2,063,800 ​ $ 6.49 ​ As of December 31, 2025, total unrecognized compensation expense related to RSUs was $ 6.2 million, which the Company expects to recognize over a weighted-average period of approximately 3.1 years. 2019 Employee Stock Purchase Plan On March 29, 2019, the Board adopted the 2019 Employee Stock Purchase Plan (the “2019 ESPP”). A total of 403,500 shares of the Company’s common stock have been reserved for issuance under the 2019 ESPP. Subject to any plan limitations, the 2019 ESPP allows eligible employees to contribute through payroll deductions up to 10 % of their earnings for the purchase of the Company’s common stock at a discounted price per share. The offering periods begin in February and August of each year, with the initial offering period starting on August 1, 2019. The common shares issuable under the 2019 ESPP were registered pursuant to a registration statement on Form S-8 on April 4, 2019. Unless otherwise determined by the administrator, the Company’s common stock will be purchased for the accounts of employees participating in the 2019 ESPP at a price per share that is the lesser of 85 % of the fair market value of the Company’s common stock on the first trading day of the offering period or 85 % of the fair market value of the Company’s common stock on the last trading day of the offering period. The fair value of estimated shares to be purchased under the 2019 ESPP is included in stock-based compensation expense. Employees have the ability to purchase shares of the Company’s common stock at a price equal to the lower of the first or last trading day of the offering period, which represents an option and, therefore, the 2019 ESPP is a compensatory plan under ASC 718-50, Employee Stock Purchase Plans . Accordingly, stock-based compensation expense is determined based on the option’s grant-date fair value, employee contributions, and the Company’s stock price and is recognized over the requisite service period of the option. The Company used the Black-Scholes valuation model to determine the fair value of ESPP. During the year ended December 31, 2025, employees purchased 71,342 shares for $ 0.3 million under the 2019 ESPP. As of December 31, 2025, there were 60,585 shares of common stock available for future issuance under the 2019 ESPP Plan. The Company recognized stock-based compensation expense related to this plan of $ 0.1 million and $ 0.2 million for the years ended December 31, 2025 and 2024, respectively. Stock-based Compensation Expense During each of the years ended December 31, 2025 and 2024, the Company recorded $ 1.0 million in net incremental stock-based compensation expense as a result of the modifications of stock awards upon termination of the former executive officers. The 2025 modification extended the option exercise period for these former executive officers by a period of six to eighteen months , and extended the vesting period by six months, whereas the 2024 modification 110 Table of Contents changed the vesting conditions of the awards by accelerating the vesting of certain unvested awards that would have otherwise been forfeited upon termination of an executive office. Stock-based compensation expense is classified in the accompanying Consolidated Statements of Operations and Comprehensive Loss for the years ended December 31, 2025 and 2024 as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2025 ​ 2024 Research and development ​ $ 6,167 ​ $ 6,351 General and administrative ​ 9,909 ​ 8,042 Total ​ $ 16,076 ​ $ 14,393 ​ ​ 11. Employee Benefit Plans The Company has a 401(k)-retirement plan in which substantially all of our employees in the United States are eligible to participate in. Eligible employees may elect to contribute up to the maximum limits, as set by the Internal Revenue Service, of their eligible compensation. During each of the years ended December 31, 2025 and 2024, the Company made discretionary plan contributions of $ 0.4 million. 12. Income Taxes The components of net loss before income tax benefit are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2025 ​ 2024 U.S. operations ​ $ ( 85,248 ) ​ $ ( 88,401 ) Non-U.S. operations ​ ( 3,526 ) ​ ( 6,658 ) Net loss before income tax benefit ​ $ ( 88,774 ) ​ $ ( 95,059 ) ​ The components of the income tax provision (benefit) consisted of the following for the years ended December 31, 2025 or 2024 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ ​ ​ 2025 ​ ​ ​ 2024 U.S. state and local ​ ​ ​ ​ Current ​ $ ( 681 ) ​ $ — Income tax expense (benefit) ​ $ ( 681 ) ​ $ — 111 Table of Contents ​ Reconciliation between the effect of applying the federal statutory rate and the effective income tax rate used to calculate the Company’s income tax benefit is as follows (in thousands, except tax rate): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2025 ​ 2024 Federal statutory rate $ ( 18,646 ) ​ 21.00 % ​ $ ( 19,962 ) ​ 21.00 % Domestic Federal ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nontaxable or nondeductible items ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Non-taxable - Section 162(m) compensation ​ ​ 1,367 ​ ( 1.50 ) ​ ​ ​ 1,087 ​ ( 1.10 ) ​ Non-taxable - Other ​ ​ 1,418 ​ ( 1.60 ) ​ ​ ​ 928 ​ ( 1.00 ) ​ Change in valuation allowance ​ ​ 14,804 ​ ( 16.70 ) ​ ​ ​ 16,546 ​ ( 17.40 ) ​ Other ​ ​ 314 ​ ( 0.40 ) ​ ​ ​ 3 ​ — ​ State income tax expense, net of federal income tax effect * ​ ​ — ​ — ​ ​ ​ — ​ — ​ Foreign ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ United Kingdom ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Credit ​ — — ​ 1,258 ​ ( 1.30 ) ​ Other ​ 362 ( 0.40 ) ​ ( 220 ) ​ 0.20 ​ Australia ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Credit ​ ​ — ​ — ​ ​ ​ — ​ — ​ Other ​ ​ 381 ​ ( 0.40 ) ​ ​ ​ 360 ​ ( 0.40 ) ​ Change in Unrecognized Tax Benefits ​ ( 681 ) 0.80 ​ ​ ​ — ​ Effective tax rate $ ( 681 ) ​ 0.80 % ​ $ — ​ ( 0.00 ) % * During the year ended December 31, 2025, state and local income taxes in Maryland make up the majority (greater than 50%) of the domestic state income taxes, net of federal effect category. During the year ended December 31, 2024, state and local income taxes in Maryland make up the majority (greater than 50%) of the domestic state income taxes, net of federal effect category. During the year December 31, 2025, the Company received a refund from the State of Maryland for $ 1.4 million. No federal, state or foreign income taxes were paid in 2025. Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income and for tax carryforwards. Significant components of the Company’s deferred tax assets and liabilities are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ ​ ​ ​ 2025 ​ ​ ​ 2024 Deferred tax assets: ​ ​ ​ ​ Net operating losses ​ $ 115,761 ​ $ 64,779 Capitalized research and development costs ​ ​ 6,936 ​ ​ 37,043 Stock compensation ​ 3,442 ​ 3,529 Accrued expenses ​ 823 ​ 726 Amortization ​ 148 ​ 199 Lease liability ​ 386 ​ 463 Depreciation ​ 41 ​ 67 Other ​ ​ 73 ​ ​ 68 Total deferred tax assets ​ 127,610 ​ 106,874 Valuation allowance ​ ( 127,141 ) ​ ( 106,423 ) Deferred tax assets, net ​ 469 ​ 451 Deferred tax liabilities: ​ ​ ​ ​ Right of use asset ​ ( 392 ) ​ ( 451 ) Other ​ ( 77 ) ​ — Total deferred tax liabilities ​ ( 469 ) ​ ( 451 ) Total deferred tax assets (liabilities), net ​ $ — ​ $ — ​ 112 Table of Contents The Company assesses the need for a valuation allowance against our deferred tax assets and considers both positive and negative evidence related to the likelihood of realization of the deferred tax assets to determine, based on the weight of available evidence, whether it is more-likely-than-not that some or all of the deferred tax assets will not be realized. This determination requires significant judgment, including assumptions about future taxable income that are based on historical and projected information. The increase in the valuation allowance during the year ended December 31, 2025 primarily relates to increases for current year losses in both the U.S. and foreign locations. The Company has recorded a valuation allowance against its net U.S. and net non-U.S. deferred tax assets which it believes are not more likely than not realizable. Deferred tax liabilities will be applied in the future to offset against net operating losses (“NOLs”) that have an indefinite life. The Company has U.S. federal and state net operating loss carryforwards of approximately $ 366.6 million and $ 365.0 million, respectively, as of December 31, 2025, of which a portion of the federal and state amount of $ 7.1 million and $ 365.0 million, respectively, has a 20-year carry forward period that will expire at various dates beginning in 2027. Under current law, the remaining federal amount of $ 359.5 million has an indefinite life and amounts utilized in the future may not exceed 80% of taxable income. The Company also has foreign net operating loss carryforward of approximately $ 59.9 million which carryforward indefinitely. Under Section 382 of the Internal Revenue Code of 1986 (“IRC 382”), as amended, substantial changes in the Company’s ownership may limit the amount of NOLs that can be utilized annually in the future to offset its U.S. federal and state taxable income. Specifically, this limitation may arise in the event of a cumulative change in ownership of the Company of more than 50 % within any three-year period. The amount of the annual limitation is determined based on the value of the Company immediately before the ownership change. The Company has reduced the NOL and related valuation allowance in historical periods for NOLs that cannot be utilized in the future because of IRC 382. The Company has reviewed for any ownership changes as defined under IRC Section 382 from January 1, 2021 through November 3, 2023 and determined that the ownership change was less than 50 % during that period. The Company’s existing NOLs are subject to limitations arising from previous ownership changes impacting the timing and amount, and the impact of such changes is reflected in the NOL amounts disclosed above. In addition, future changes in the Company’s stock ownership, many of which are outside of the Company’s control, could result in an ownership change. On July 4, 2025, the One Big Beautiful Bill Act (“OBBBA”), which includes a broad range of tax reform provisions, was signed into law in the United States, which includes a new Internal Revenue Code ("IRC") Section 174A. Under Section 174A, commencing with tax years beginning after December 31, 2024, domestic research or experimental expenditures may be deducted in the current period rather than capitalized and amortized over multiple years, as previously required under IRC Section 174. The OBBBA does not have a material impact on our effective tax rate, financial condition, or results of operations in 2025. Significant judgment is required in evaluating tax positions and determining the provision for income taxes. The Company establishes liabilities for tax-related uncertainties based on estimates of whether, and the extent to which, additional taxes may be due. These liabilities are established when the Company believes that its tax return positions are more-likely-than-not to be sustained upon audit by taxing authorities. The Company adjusts these liabilities in light of changing facts and circumstances, such as the outcome of a tax audit. The provision for income taxes includes the impact of changes to these liabilities. The Company had $ 0.7 million in unrecognized tax benefits as of December 31, 2024. Any changes in the next twelve months are not anticipated to have a significant impact on the results of operations, financial position or cash flows of the Company. The Company had no remaining uncertain tax positions as of December 31, 2025. 113 Table of Contents The following is a tabular reconciliation of the total amounts of unrecognized tax benefits excluding related interest and penalties (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2025 ​ 2024 Beginning balance ​ $ 711 ​ $ 711 Decreases for prior year tax positions ​ ( 711 ) ​ — Ending balance ​ $ — ​ $ 711 ​ The Company files income tax returns in the United States, various U.S. states, U.K. and Australia. The Company is still open to examination by the applicable taxing authorities from 2021 forward, although tax attributes that were generated prior to 2021 may still be adjusted upon examination by federal, state, foreign or local tax authorities if they either have been or will be used in a future period. 13. Net Loss Per Share Because the Company has reported net loss attributable to common stockholders for the years ended December 31, 2025 and 2024, basic and diluted net loss per share attributable to common stockholders in each year are the same. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted-average numbers of shares of common stock outstanding for the period. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period. As such, all unvested RSUs, common stock warrants, and stock options have been excluded from the computation of diluted weighted-average shares outstanding because such securities would have an anti-dilutive impact for all periods presented. Potential common shares issuable upon conversion, vesting or exercise of unvested RSUs, common stock warrants, and stock options that are excluded from the computation of diluted weighted-average shares outstanding, as they are anti-dilutive, are as follows: ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2025 ​ 2024 Common stock options 11,348,778 6,666,131 Restricted stock units 2,063,800 ​ 858,290 ​ ​ 14. Commitments and Contingencies Spitfire Acquisition In July 2019, the Company entered into the Spitfire merger agreement to acquire all of the equity interests of Spitfire Pharma, Inc. (“Spitfire”). Spitfire was a privately held, preclinical pharmaceutical company developing novel peptide products for pharmaceutical indications, including pemvidutide for the treatment of MASH. As part of the agreement, the Company is obligated to make payments of up to $ 80.0 million upon the achievement of specified worldwide net sales of all products developed using the technology acquired from Spitfire Pharma Inc. (the “Sales Milestone”) within ten years following the approval of a new drug application filed with the U.S. Food and Drug Administration (the “FDA”). The contingent payments related to the Sales Milestones are predominately cash-based payments accounted for under FASB Accounting Standards Codification Topic 450, Contingencies. Accordingly, the Company will recognize the Sales Milestones when the contingency is probable and the amount can be reasonably estimated. 114 Table of Contents Litigation On August 5, 2025, a class action complaint was filed in federal district court in the District of Maryland, Southern Division, naming as defendants the Company and two of the Company’s executive officers, which was captioned In re Altimmune, Inc. Securities Litigation , Case No. 8:25-cv-02581 (D. Md.) (the “Securities Class Action”). The Securities Class Action alleged that the defendants violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 thereunder, by making false and misleading statements and omissions of material fact to the investing public including the plaintiff and class members, who purchased or otherwise acquired the Company’s common stock between August 10, 2023 and June 25, 2025, including relating to pemvidutide and our IMPACT Phase 2b trial of pemvidutide in MASH. The plaintiff and class members sought, among other things, to have the action maintained as a class action under Rule 23 of the Federal Rules of Civil Procedure and for the defendants to pay damages, interest, and an award of costs, including attorneys’ fees. On October 16, 2025, the court appointed lead plaintiffs for the litigation. On October 29, 2025, the court entered an order requiring lead plaintiffs to file an amended complaint by November 26, 2025, and for defendants to file an answer or notice of intent to file a motion to dismiss such amended complaint by December 10, 2025. Rather than amend, the lead plaintiffs sought voluntary dismissal without prejudice of the action, which the Court granted on November 26, 2025. On September 29, 2025, a shareholder derivative complaint was filed in federal district court in the District of Maryland, purportedly on behalf of the Company, naming as defendants two of the Company’s then-executive officers and eight of the Company’s current and former board members, captioned Alaraidah v. Altimmune, Inc., et al. , No. 8:25-cv-03223 (D. Md.) (the “Derivative Action”). The complaint was based upon allegations that were similar to those alleged in a class action complaint previously filed against us in the Securities Class Action and alleged claims for violations of Section 14(a) of the Securities Exchange Act of 1934, breaches of fiduciary duty, unjust enrichment, abuse of control, gross mismanagement, waste of corporate assets, and contribution under Sections 10(b) and 21D of the Securities Exchange Act of 1934 based on the defendants purportedly making or causing to be made false and misleading statements and omissions of material fact between August 10, 2023 and June 25, 2025. The complaint sought unspecified monetary relief, restitution, costs, and equitable relief. On December 22, 2025, the parties filed a stipulation of dismissal without prejudice, which the Court granted on that date. The Company is a party in various contracts that are subject to potential disputes, litigation, and claims arising in the ordinary course of business, none of which are currently reasonably possible or probable of material loss. ​ 15. Segment Information The Company is a late clinical-stage biopharmaceutical company developing novel therapies for serious liver diseases. The Company’s lead product candidate is pemvidutide, a balanced 1:1 glucagon/GLP-1 dual receptor agonist in development for the treatment of MASH, AUD and ALD. To date, the Company has not generated any revenue from the sale of any products. As described in Note 2, the Chief Executive Officer is the chief operating decision maker (CODM). The CODM assesses the performance of the Company and decides how to allocate resources based solely on net (loss) income, which is also reported on the consolidated statements of operations and consolidated loss as net (loss) income. The measure of segment assets is reported on the consolidated balance sheet as total assets. ​ ​ 16. Subsequent Events The Company evaluated subsequent events through the issuance date of the financial statements. During January and February 2026, the Company raised $ 8.2 million in net proceeds through the issuance of 1,874,597 shares of common stock from the Company’s at-the-market offerings program under the November 2025 Agreement. On January 27, 2026, the Company entered into a securities purchase agreement with a new fundamental institutional investor pursuant to a registered direct offering under the November 2025 Shelf for the purchase and sale of 12,397,920 shares of its common stock and 4,647,534 pre-funded warrants for net proceeds of approximately $ 70.4 115 Table of Contents million. The pre-funded warrants were fully exercised on February 13, 2026, resulting in the issuance of 4,647,534 shares of the Company’s common stock. ​ ​ 116 Table of Contents Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure None.