What vaccine strains were included in the 2025 WHO influenza vaccine recommendation?

1. What the WHO actually named — precise strain list that matters to manufacturers and regulators

The WHO recommended specific virus strains for use in Northern Hemisphere vaccines for the 2025–2026 season: for egg-based trivalent vaccines the components are A/Victoria/4897/2022 (H1N1)pdm09-like, A/Croatia/10136RV/2023 (H3N2)-like, and B/Austria/1359417/2021 (B/Victoria lineage)-like. For cell culture-, recombinant protein- or nucleic acid-based trivalent vaccines the recommended components are A/Wisconsin/67/2022 (H1N1)pdm09-like, A/District of Columbia/27/2023 (H3N2)-like, and B/Austria/1359417/2021 (B/Victoria lineage)-like. These recommendations were published in a WHO technical document dated 28 February 2025 ^{[1] [4]}.

2. Why two different H1N1 and H3N2 picks — egg adaptation and product platforms

WHO explicitly split the H1N1 and H3N2 component choices by vaccine production platform because egg propagation can select for antigenic changes that differ from viruses grown in cell culture or expressed by recombinant/nucleic acid methods, so distinct candidate viruses optimize match for each manufacturing pathway. The egg-based H1N1 recommended virus is A/Victoria/4897/2022 while the cell/recombinant recommendation is A/Wisconsin/67/2022; similarly, egg-based H3N2 is A/Croatia/10136RV/2023 and the cell-based H3N2 recommendation is A/District of Columbia/27/2023, reflecting platform-specific selection to maintain antigenic fidelity ^{[1] [4]}.

3. The B lineage decision — continuity on B/Victoria, status of B/Yamagata

The WHO maintained B/Austria/1359417/2021 (B/Victoria lineage)-like virus as the recommended B component across trivalent and quadrivalent vaccines. For quadrivalent vaccines, the B/Yamagata lineage component remains B/Phuket/3073/2013-like, a carryover from prior seasons; WHO has continued this inclusion despite limited global circulation of Yamagata-lineage viruses, reflecting conservative continuity in quadrivalent composition ^{[1] [2] [4]}.

4. Pandemic preparedness advice — new avian H5 strain recommendations included

In addition to seasonal vaccine composition, WHO advisers recommended candidate vaccine viruses for avian influenza preparedness, specifically H5N1 clade 2.3.2.1a and H5N6 clade 2.3.4.4h strains. These pandemic-preparedness picks are separate from seasonal vaccine composition but were issued alongside the seasonal recommendations to help manufacturers and regulators prepare reagents and stockpiles in case of an avian-derived human outbreak ^[3].

5. How WHO framed the change — match, surveillance, and regulatory uptake

WHO advisers described the H3N2 selection as a targeted update in response to global virological surveillance and antigenic characterization; H3N2 remains the most antigenically variable component, making periodic adjustment necessary for vaccine effectiveness. WHO’s technical recommendation is advisory: national regulatory authorities and manufacturers will use this guidance to update licensed vaccines and production strains for the 2025–2026 season, with the WHO document serving as the authoritative composition reference dated 28 February 2025 ^{[3] [1] [2]}.

6. Limitations, continuity and what to watch next — transparency and seasonal evolution

The WHO recommendations replicate prior-season continuity for some components while changing others, balancing stability for production with antigenic updates driven by surveillance. The same B/Victoria strain is retained, the B/Yamagata quadrivalent component persists, and platform-specific H1N1/H3N2 choices reflect manufacturing realities. National roll-out timing, actual vaccine brands, and real-world vaccine effectiveness will depend on manufacturing timelines and circulating virus drift during the coming season; WHO’s guidance is dated 28 February 2025 and should be read as the definitive composition for the Northern Hemisphere 2025–2026 influenza vaccine season ^{[1] [5]}.