Which influenza A(H1N1) and A(H3N2) vaccine strains were selected for the 2025-26 northern hemisphere formulation?
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Executive summary
For the 2025–26 northern hemisphere influenza season, health authorities recommended trivalent vaccines containing an A(H1N1) pdm09–like virus, an A(H3N2)–like virus, and a B/Victoria–lineage virus. U.S. (egg‑based) recommendations named A/Victoria/4897/2022 (H1N1)pdm09–like and A/Croatia/10136RV/2023 (H3N2)–like strains; cell/recombinant U.S. recommendations named A/Wisconsin/67/2022 (H1N1)pdm09–like and A/District of Columbia/27/2023 (H3N2)–like strains [1] [2] [3]. WHO and EU guidance likewise endorsed trivalent composition for 2025–26 [4] [5] [6].
1. What strains were selected — the short answer
U.S. egg‑based vaccines: A/Victoria/4897/2022 (H1N1)pdm09‑like virus and A/Croatia/10136RV/2023 (H3N2)‑like virus, plus B/Austria/1359417/2021 (B/Victoria lineage) [1]. U.S. cell‑ or recombinant‑based vaccines: A/Wisconsin/67/2022 (H1N1)pdm09‑like virus and A/District of Columbia/27/2023 (H3N2)‑like virus, plus the same B/Austria/1359417/2021 (B/Victoria lineage) [1] [2]. WHO published its recommended composition for the northern hemisphere consistent with trivalent formulations [4] [5].
2. Why there are different H1N1/H3N2 entries for egg vs cell/recombinant vaccines
Regulatory panels selected strains by manufacturing method because egg‑adaptation can change viral antigens and affect vaccine match; manufacturers therefore use candidate strains best suited for egg propagation versus cell/recombinant platforms (FDA statement and advisory materials describe separate selections for egg‑based and cell/recombinant vaccines) [2] [1]. The FDA explicitly recommended different H1N1 and H3N2 representatives for the two production paths [2] [1].
3. The move to trivalent formulations and the B strain choice
For 2025–26 authorities reverted to—or maintained—trivalent vaccines containing two A subtypes and one B/Victoria lineage virus; B/Austria/1359417/2021 (B/Victoria) was chosen across recommended products in the U.S. guidance [1] [3]. The European Medicines Agency and WHO documents note a shift away from routinely including B/Yamagata after its apparent absence, which has driven practical moves toward trivalent formulations for 2025–26 [6] [5].
4. How these selections were reached — surveillance and advisory process
Recommendations followed comprehensive review of U.S. and global surveillance and antigenic/genetic analyses presented to advisory bodies; FDA worked with CDC, Department of Defense, and international partners to finalize recommendations timely for manufacturing [2] [1] [7]. WHO’s Global Influenza Programme provided the broader international recommendations that inform regional regulators [4] [5].
5. What this means for vaccine effectiveness and uncertainties
Available sources report that vaccine effectiveness depends on antigenic match and manufacturing method; preliminary Southern Hemisphere 2025 data suggested about half protection overall and varying effectiveness by strain, indicating reasonable protection but not complete prevention [8] [9]. Early US and UK reports later in 2025 suggested vaccine effectiveness can be substantially different by age and strain, and observers warned that H3N2 mismatches can reduce protection in older adults [10] [8]. Sources do not provide final 2025–26 Northern Hemisphere VE estimates in this dataset — not found in current reporting [10] [8].
6. Competing perspectives and potential agendas in the reporting
Regulators emphasize data‑driven choices and manufacturing constraints (FDA, WHO, EMA communications) [2] [4] [6]. Industry statements stress preparedness and product availability — Sanofi highlighted adoption of FDA‑selected strains to ensure supply [11]. Pharmacy Times reported internal‑panel decisions and emphasized an H2N2 mention that is inconsistent with other official sources in this set; that article’s claim about inclusion of A/H2N2 in 2025–26 diverges from FDA/WHO/EMA summaries and should be treated cautiously (p1_s2 versus [1]; p1_s3).
7. Bottom line for clinicians, public health officials and the public
The 2025–26 northern hemisphere vaccines were specified as trivalent with distinct strain choices depending on egg versus cell/recombinant manufacture: key A(H1N1) and A(H3N2) candidates were A/Victoria/4897/2022 and A/Croatia/10136RV/2023 for egg vaccines, and A/Wisconsin/67/2022 and A/District of Columbia/27/2023 for cell/recombinant vaccines, alongside B/Austria/1359417/2021 (B/Victoria) [1] [2] [3]. Those planning vaccine programs should follow the specific product labeling and manufacturer guidance because antigen selection varies by production method [2] [1].
Limitations: this analysis uses only the supplied documents; WHO’s full recommendation text and final region‑specific licensure materials may add detail not covered here — available sources do not mention any further strain names beyond those cited [4] [1].