Altimmune, Inc. Stock Compensation Disclosure

9. Stock-Based Compensation

Stock Options

The Company established the 2001 Employee Stock Option Plan to provide incentive stock options and non-qualified stock options to employees, and the 2001 Non-employee Stock Option Plan to provide non-qualified stock options to the members of the board of directors and advisory board, and non-employees. The 2001 Employee Stock Option Plan and the 2001 Non-employee Stock Option Plan are collectively referred to as the “2001 Plans.” In connection with the Company’s merger with PharmAthene, Inc. in 2017, the Company issued options from its 2001 Plans to replace options previously granted. The Company de-designated common stock available for issuance under the 2001 Plans. No additional options or restricted stock will be granted under these plans. Options outstanding and unvested restricted stock granted or replaced under these plans will continue to vest over the remaining vesting period through the earlier of exercise, expiration or forfeiture. The replacement options issued after the 2017 mergers will continue to vest over the remaining vesting period through the earlier of exercise, expiration or forfeiture. Also, in connection with the 2017 mergers, the 2001 Plans were assumed by the Company.

In addition, the Company assumed the PharmAthene, Inc. Amended and Restated 2007 Long-Term Incentive Compensation Plan (the “2007 Plan”). Awards outstanding under the 2007 Plan remained outstanding in accordance with their applicable terms and conditions. No additional awards will be made under the 2007 Plan.

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, 2025, the number of shares of Common Stock reserved and available for issuance under the Omnibus Plan increased by 3,193,659. 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 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 2001 Plans, the 2007 Plan, the Omnibus Plan and the Inducement Plan are collectively referred to as the “Plans.” During the year ended December 31, 2024 under the Plans, a total of 2,106,375 options to purchase shares of common stock were granted. As of December 31, 2024, there were 2,637,161 and 1,063,695 shares of common stock available for future grants under the Omnibus Plan and the Inducement Plan, respectively.

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:

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A summary of stock option activity under the Plans is presented below (in thousands, except share and per share data):

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The per share weighted-average grant date fair value of stock options granted during the years ended December 31, 2024 and 2023 were $7.07 and $7.86 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2024 and 2023 was $1.1 million and $0.2 million, respectively. The total fair value of options vested during the years ended December 31, 2024 and 2023 was $11.6 million and $7.0 million, respectively. As

of December 31, 2024, there was $17.5 million of unrecognized compensation cost related to stock options, which is expected to be recognized over a weighted-average period of 2.5 years.

Restricted Stock Units (RSUs)

During the year ended December 2023, the Company granted 559,600 shares of RSUs with a weighted-average grant date fair value of $9.05 per share which vest over four years. During the year ended December 31, 2024, the Company issued 192,537 shares of common stock as a result of the vesting of 288,977 RSUs net of 96,440 shares of common stock withheld to satisfy tax withholding obligations. The total fair value of RSUs vested during the years ended December 31, 2024 and 2023 was $2.5 million and $1.4 million, respectively.

A summary of RSUs activities is presented below:

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As of December 31, 2024, total unrecognized compensation expense related to RSUs was $5.4 million, which the Company expects to recognize over a weighted-average period of approximately 2.5 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 started 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 2019 ESPP estimated shares to be purchased fair value 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, 2024, employees purchased 86,857 shares for $0.3 million under the 2019 ESPP. As of December 31, 2024, there were 131,927 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 $0.2 million for each of the years ended December 31, 2024 and 2023.

Stock-based Compensation Expense

During the year ended December 31, 2024, the Company recorded $1.0 million incremental stock-based compensation expense as a result of the modifications of stock awards upon termination of the former Chief Financial Officer. The modification changed the vesting conditions of the awards by accelerating the vesting of certain unvested awards that would have otherwise been forfeited upon termination. The incremental costs are included in the Statements of Operations and Comprehensive Loss under general and administrative expense.

Stock-based compensation expense is classified in the accompanying Consolidated Statements of Operations and Comprehensive Loss for the years ended December 31, 2024 and 2023 as follows (in thousands):

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