Altimmune, Inc.

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 andoutstanding.

Each share of common stock entitles the holder toone 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 hadno 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

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.