What influenza A(H1N1) strain did WHO recommend for 2025?

1. Why the answer is not a single strain — WHO split guidance and what that means

WHO issued separate A(H1N1) recommendations tied to vaccine production method: egg-based manufacturers should use A/Victoria/4897/2022 (H1N1)pdm09-like, while cell culture-, recombinant protein- and nucleic acid-based manufacturers should use A/Wisconsin/67/2022 (H1N1)pdm09-like for the 2025–2026 northern hemisphere season, reflecting differences in antigenic behavior and production yields between platforms ^{[1] [2]}. WHO’s meeting notes and composition announcements routinely distinguish strains by production pathway because egg adaptation can alter antigenic properties, so recommending different seed strains seeks to optimize vaccine match and immune response across technologies ^{[1] [2]}. The recommendation therefore answers “what H1N1 strain for 2025” with a conditional response tied to vaccine manufacturing, not a single universal isolate.

2. Cross-checking across WHO announcements — consistency and timing

WHO’s February 2025 composition recommendation for the northern hemisphere cites the twin H1N1 choices above, and earlier WHO materials for the 2024–2025 season showed very similar dual-strain patterns (A/Victoria/4897/2022 for eggs and A/Wisconsin/67/2022 for cell/recombinant), indicating continuity rather than abrupt change between seasons ^{[4] [1]}. By contrast, a separate WHO announcement for the 2026 southern hemisphere season later named A/Missouri/11/2025 (H1N1)pdm09-like for both egg- and cell-based vaccines, demonstrating WHO’s annual reappraisal and the fact that the 2026 southern choice postdates and should not be used to answer “what was recommended for 2025” ^[3]. The timeline shows distinct, date-specific recommendations rather than a single enduring strain.

3. How other agencies aligned — U.S. FDA and international harmonization

U.S. regulators and advisory bodies reflected the WHO distinction for the 2025–2026 U.S. season: the FDA’s composition advice mirrored WHO’s split, recommending A/Victoria/4897/2022 for egg-based vaccines and A/Wisconsin/67/2022 for cell- or recombinant-based vaccines, which aligns U.S. practice with WHO’s platform-specific approach ^[5]. This alignment suggests global surveillance and regulatory review converged on the same pragmatic solution to platform-induced antigenic differences for that season ^{[2] [5]}. The agreement also underscores that vaccine producers followed platform-specific seeds rather than a single universal H1N1 isolate for 2025–2026 formulation.

4. Common confusion — southern vs northern hemisphere timing and strain names

Public and media confusion often arises when WHO issues multiple, time-stamped seasonal recommendations: the 2026 southern hemisphere recommendation (A/Missouri/11/2025) was issued in late 2025 and refers to vaccines for the southern hemisphere 2026 season, not the 2025 northern-season composition ^[3]. Answering “what influenza A(H1N1) strain did WHO recommend for 2025?” therefore requires clarifying the season and production method; the correct 2025–2026 northern-season response is the split A/Victoria/4897/2022 and A/Wisconsin/67/2022 guidance, while the later southern-season pick differs and reflects subsequent surveillance ^{[1] [3]}.

5. Bottom line and how to read future recommendations

For the 2025–2026 northern hemisphere vaccine composition, WHO recommended A/Victoria/4897/2022 for egg-based vaccines and A/Wisconsin/67/2022 for cell culture-, recombinant protein- or nucleic acid-based vaccines; a later 2026 southern-season recommendation named A/Missouri/11/2025 and should not be retroactively applied to 2025 guidance ^{[1] [3]}. Because WHO updates compositions annually based on global surveillance, the most reliable answer for any season is the date-stamped WHO composition statement and the paragraph-specific distinction by vaccine production route; users seeking the current recommended A(H1N1) strain must check the WHO composition notice dated for the season in question. ^{[1] [3]}