X0202 SCHEDULE 13G 0001103804 XXXXXXXX LIVE Common Stock, par value $0.0001 per share 04/24/2026 0001326190 Altimmune, Inc. 02155H200 910 Clopper Road Suite 201S Gaithersburg MD 20878 Rule 13d-1(c) VIKING GLOBAL INVESTORS LP b DE 0 13400000 0 13400000 13400000 N 6.7 PN See Item 4 Viking Global Opportunities Parent GP LLC b DE 0 13400000 0 13400000 13400000 N 6.7 OO See Item 4 Viking Global Opportunities GP LLC b DE 0 9380000 0 9380000 9380000 N 4.7 OO See Item 4 Viking Global Opportunities Portfolio GP LLC b DE 0 9380000 0 9380000 9380000 N 4.7 OO See Item 4 Viking Global Opportunities Illiquid Investments Sub-Master LP b E9 0 9380000 0 9380000 9380000 N 4.7 PN See Item 4 Viking Global Opportunities Vintage IV GP LLC b DE 0 4020000 0 4020000 4020000 N 2 OO See Item 4 Viking Global Opportunities Vintage IV Portfolio GP LLC b DE 0 4020000 0 4020000 4020000 N 2 OO See Item 4 Viking Global Opportunities Vintage IV (Aggregator) LP b E9 0 4020000 0 4020000 4020000 N 2 PN See Item 4 HALVORSEN OLE ANDREAS b Q8 0 13400000 0 13400000 13400000 N 6.7 IN See Item 4 Shabet Rose Sharon b X1 0 13400000 0 13400000 13400000 N 6.7 IN See Item 4 Altimmune, Inc. 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878 Viking Global Investors LP ("VGI"), Viking Global Opportunities Parent GP LLC ("Opportunities Parent"), Viking Global Opportunities GP LLC ("Opportunities GP"), Viking Global Opportunities Portfolio GP LLC ("Opportunities Portfolio GP"), Viking Global Opportunities Illiquid Investments Sub-Master LP ("VGOP"), Viking Global Opportunities Vintage IV GP LLC ("VGOVIV GP"), Viking Global Opportunities Vintage IV Portfolio GP LLC ("VGOVIV Portfolio GP"), Viking Global Opportunities Vintage IV (Aggregator) LP ("VGOVIV"), O. Andreas Halvorsen and Rose S. Shabet (collectively, the "Reporting Persons") The business address of each of the Reporting Persons is: 600 Washington Boulevard, Floor 11, Stamford, Connecticut 06901. VGI is a Delaware limited partnership; Opportunities Parent, Opportunities GP, Opportunities Portfolio GP, VGOVIV GP and VGOVIV Portfolio GP are Delaware limited liability companies; VGOP and VGOVIV are Cayman Islands exempted limited partnerships; O. Andreas Halvorsen is a citizen of Norway; and Rose S. Shabet is a citizen of the United States. Y This Statement on Schedule 13G relates to shares of Common Stock and warrants to purchase shares of Common Stock at an exercise price of $3.00 per share (the "Warrants") held directly by VGOP and VGOVIV. VGI: 13,400,000 VGI provides managerial services to VGOP and VGOVIV. VGI has the authority to dispose of and vote the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants. VGI does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the "Act"), VGI may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants. VGI beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants. Opportunities Parent: 13,400,000 Opportunities Parent is the sole member of Opportunities GP, which has the authority to dispose of and vote the shares of Common Stock controlled by Opportunities Portfolio GP (which consists of the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants), and is the sole member of VGOVIV GP, which has the authority to dispose of and vote the shares of Common Stock controlled by VGOVIV Portfolio GP (which consists of the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants). Opportunities Parent does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, Opportunities Parent may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants. Opportunities Parent beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants. Opportunities GP: 9,380,000 Opportunities GP serves as the sole member of Opportunities Portfolio GP and has the authority to dispose of and vote the shares of Common Stock controlled by Opportunities Portfolio GP, which consists of the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. In addition, Opportunities GP is the general partner of each of Viking Global Opportunities Intermediate LP, Viking Global Opportunities III LP, and Viking Global Opportunities LP. Opportunities GP does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, Opportunities GP may be deemed to beneficially own the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. Opportunities GP beneficially owns 9,380,000 shares of Common Stock consisting of 4,690,000 shares of Common Stock directly owned by VGOP and 4,690,000 shares of Common Stock beneficially owned by VGOP, which VGOP has the right to purchase upon exercise of the Warrants. Opportunities Portfolio GP: 9,380,000 Opportunities Portfolio GP serves as the general partner of VGOP and has the authority to dispose of and vote the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. Opportunities Portfolio GP does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, Opportunities Portfolio GP may be deemed to beneficially own the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. Opportunities Portfolio GP beneficially owns 9,380,000 shares of Common Stock consisting of 4,690,000 shares of Common Stock directly owned by VGOP and 4,690,000 shares of Common Stock beneficially owned by VGOP, which VGOP has the right to purchase upon exercise of the Warrants. VGOP: 9,380,000 VGOP has the authority to dispose of and vote the 4,690,000 shares of Common Stock that it owns and the 4,690,000 shares of Common Stock it has the right to purchase upon exercise of the Warrants directly owned by it, which power may be exercised by its general partner, Opportunities Portfolio GP, and by VGI, an affiliate of Opportunities Portfolio GP, which provides managerial services to VGOP. Viking Global Opportunities LP (a Delaware limited partnership) and Viking Global Opportunities III LP (a Cayman Islands exempted limited partnership), through its investment in Viking Global Opportunities Intermediate LP (a Cayman Islands exempted limited partnership), invest substantially all of their assets in Viking Global Opportunities Master LP (a Cayman Islands exempted limited partnership), which in turn invests through VGOP. VGOVIV GP: 4,020,000 VGOVIV GP serves as the sole member of VGOVIV Portfolio GP and has the authority to dispose of and vote the shares of Common Stock controlled by VGOVIV Portfolio GP, which consists of the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. In addition, VGOVIV GP is the general partner of each of Viking Global Opportunities Vintage IV (Offshore) LP and Viking Global Opportunities Vintage IV (Onshore) LP. VGOVIV GP does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, VGOVIV GP may be deemed to beneficially own the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. VGOVIV GP beneficially owns 4,020,000 shares of Common Stock consisting of 2,010,000 shares of Common Stock directly owned by VGOVIV and 2,010,000 shares of Common Stock beneficially owned by VGOVIV, which VGOVIV has the right to purchase upon exercise of the Warrants. VGOVIV Portfolio GP: 4,020,000 VGOVIV Portfolio GP serves as the general partner of VGOVIV and has the authority to dispose of and vote the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. VGOVIV Portfolio GP does not directly own any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, VGOVIV Portfolio GP may be deemed to beneficially own the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. VGOVIV Portfolio GP beneficially owns 4,020,000 shares of Common Stock consisting of 2,010,000 shares of Common Stock directly owned by VGOVIV and 2,010,000 shares of Common Stock beneficially owned by VGOVIV, which VGOVIV has the right to purchase upon exercise of the Warrants. VGOVIV: 4,020,000 VGOVIV has the authority to dispose of and vote the 2,010,000 shares of Common Stock that it directly owns and the 2,010,000 shares of Common Stock it has the right to purchase upon exercise of the Warrants directly owned by it, which power may be exercised by VGOVIV Portfolio GP, and by VGI, an affiliate of VGOVIV Portfolio GP, which provides managerial services to VGOVIV. Viking Global Opportunities Vintage IV (Offshore) LP (a Cayman Islands exempted limited partnership) and Viking Global Opportunities Vintage IV (Onshore) LP (a Delaware limited partnership) invest substantially all of their assets in VGOVIV. O. Andreas Halvorsen and Rose S. Shabet: 13,400,000 Mr. Halvorsen and Ms. Shabet, as Executive Committee Members of Viking Global Partners LLC (general partner of VGI) and Opportunities Parent, have shared authority to dispose of and vote the shares of Common Stock beneficially owned by VGI and Opportunities Parent. Neither Mr. Halvorsen nor Ms. Shabet directly owns any shares of Common Stock or Warrants. Based on Rule 13d-3 under the Act, each may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants. Each of Mr. Halvorsen and Ms. Shabet beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants. The percentages set forth herein are based on (i) 194,199,358 shares of Common Stock outstanding following the Issuer's underwritten offering that closed on April 24, 2026, as reported in the Issuer's prospectus on Form 424B5, filed with the Securities and Exchange Commission (the "Commission") on April 24, 2026, and (ii) 6,700,000 shares of Common Stock that the Reporting Persons currently have the right to purchase upon exercise of the Warrants. VGI: 6.7% Opportunities Parent: 6.7% Opportunities GP: 4.7% Opportunities Portfolio GP: 4.7% VGOP: 4.7% VGOVIV GP: 2.0% VGOVIV Portfolio GP: 2.0% VGOVIV: 2.0% O. Andreas Halvorsen and Rose S. Shabet: 6.7% VGI: 0 Opportunities Parent: 0 Opportunities GP: 0 Opportunities Portfolio GP: 0 VGOP: 0 VGOVIV GP: 0 VGOVIV Portfolio GP: 0 VGOVIV: 0 O. Andreas Halvorsen and Rose S. Shabet: 0 VGI: 13,400,000 Opportunities Parent: 13,400,000 Opportunities GP: 9,380,000 Opportunities Portfolio GP: 9,380,000 VGOP: 9,380,000 VGOVIV GP: 4,020,000 VGOVIV Portfolio GP: 4,020,000 VGOVIV: 4,020,000 O. Andreas Halvorsen and Rose S. Shabet: 13,400,000 VGI: 0 Opportunities Parent: 0 Opportunities GP: 0 Opportunities Portfolio GP: 0 VGOP: 0 VGOVIV GP: 0 VGOVIV Portfolio GP: 0 VGOVIV: 0 O. Andreas Halvorsen and Rose S. Shabet: 0 VGI: 13,400,000 Opportunities Parent: 13,400,000 Opportunities GP: 9,380,000 Opportunities Portfolio GP: 9,380,000 VGOP: 9,380,000 VGOVIV GP: 4,020,000 VGOVIV Portfolio GP: 4,020,000 VGOVIV: 4,020,000 O. Andreas Halvorsen and Rose S. Shabet: 13,400,000 Y N The response to Item 4 is incorporated by reference herein. Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. EXHIBIT A - JOINT FILING AGREEMENT VIKING GLOBAL INVESTORS LP /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Parent GP LLC /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities GP LLC /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Portfolio GP LLC /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Illiquid Investments Sub-Master LP /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Vintage IV GP LLC /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Vintage IV Portfolio GP LLC /s/ Scott M. Hendler 05/01/2026 Viking Global Opportunities Vintage IV (Aggregator) LP /s/ Scott M. Hendler 05/01/2026 HALVORSEN OLE ANDREAS /s/ Scott M. Hendler 05/01/2026 Shabet Rose Sharon /s/ Scott M. Hendler 05/01/2026 (1) Scott M. Hendler is signing on behalf of O. Andreas Halvorsen, individually and as an Executive Committee Member of VIKING GLOBAL PARTNERS LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES PARENT GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES GP LLC, VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC, VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP, VIKING GLOBAL OPPORTUNITIES VINTAGE IV GP LLC, VIKING GLOBAL OPPORTUNITIES VINTAGE IV PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES VINTAGE IV (AGGREGATOR) LP, pursuant to an authorization and designation letter dated February 9, 2021, which was previously filed with the Commission as an exhibit to a Form 13G filed by Mr. Halvorsen on February 12, 2021 (SEC File No. 005-49737). (2) Scott M. Hendler is signing on behalf of Rose S. Shabet, individually and as an Executive Committee Member of VIKING GLOBAL PARTNERS LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES PARENT GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES GP LLC, VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC, VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP, VIKING GLOBAL OPPORTUNITIES VINTAGE IV GP LLC, VIKING GLOBAL OPPORTUNITIES VINTAGE IV PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES VINTAGE IV (AGGREGATOR) LP, pursuant to an authorization and designation letter dated February 9, 2021, which was previously filed with the Commission as an exhibit to a Form 13G filed by Mr. Halvorsen on February 12, 2021 (SEC File No. 005-49737).