Have WHO or national agencies updated vaccine strain recommendations for 2025-2026 H3N2?
Executive summary
WHO’s advisory group recommended A/Croatia/10136RV/2023-like as the A(H3N2) component for Northern Hemisphere 2025–26 trivalent (egg-based) vaccines; WHO also selected different H3N2 candidates for Southern Hemisphere 2026 egg- and cell-based vaccines (A/Singapore/GP20238/2024-like for egg, A/Sydney/1359/2024-like for cell) [1] [2]. Since vaccine strain selection, a genetically distinct H3N2 “subclade K” emerged and grew rapidly, accounting for roughly one-third of global H3N2 sequences between May–Nov 2025 and raising concerns about a mismatch with the WHO-picked H3N2 vaccine strain [3] [4].
1. WHO’s formal recommendation for NH 2025–26: what was chosen
The World Health Organization’s published recommendation for the 2025–26 Northern Hemisphere trivalent vaccine specifies an A/Croatia/10136RV/2023-like virus as the A(H3N2) egg‑based component [1]. WHO documents and candidate-virus listings show the A/Croatia reference was the formal pick used to guide manufacturers for the 2025–26 Northern Hemisphere production cycle [5] [1].
2. Southern Hemisphere and egg vs. cell divergence: WHO later changed some picks
At the Southern Hemisphere vaccine composition meeting, WHO advisers replaced both H1N1 and H3N2 picks for the 2026 Southern Hemisphere formulations, and they set different H3N2 candidates for egg‑ and cell‑based vaccines — recommending A/Singapore/GP20238/2024-like for egg-derived vaccines and A/Sydney/1359/2024-like for cell-derived vaccines [2]. The WHO report for the SH meeting and accompanying technical material document these separate picks and explain antigenic differences seen in circulating viruses [5] [2].
3. The emergence of subclade K after strain selection: timing and scope
Multiple surveillance and reporting threads say the subclade K variant (formerly J.2.4.1) acquired several HA mutations in mid‑2025, after WHO strain selection for manufacturers, and then expanded rapidly; ECDC and global surveillance data indicate subclade K accounted for about one‑third of A(H3N2) sequences deposited to GISAID from May–November 2025 and nearly half of EU sequences in that period [3] [4]. Reporting by news outlets and public‑health analysis corroborates that subclade K arose months after vaccine reference viruses were chosen [6] [7].
4. Do WHO or national agencies “update” mid‑season? What the sources show
Available sources show WHO sets formal composition recommendations twice yearly (Northern and Southern Hemisphere meetings) and issued the A/Croatia/10136RV/2023-like recommendation for NH 2025–26 and the SH 2026 changes noted above [1] [2]. Sources document antigenic concerns about subclade K and national risk assessments (ECDC technical brief) but do not show WHO or major national agencies issuing a mid‑season replacement to change the already‑produced 2025–26 H3N2 vaccine component after manufacturers began production [5] [3]. In short, WHO published the recommendations; subsequent surveillance flagged a divergent K subclade but the formal vaccine composition for NH 2025–26 remained as published [1] [3].
5. Evidence on mismatch and real‑world impact: competing viewpoints
Genetic and antigenic analyses cited by ECDC and news reporting say subclade K is antigenically divergent from the A/Croatia vaccine reference and that post‑vaccination sera had lower reactivity to several J.2 group subclades [3] [5]. Media and analysis pieces warn this could lower vaccine effectiveness and have documented early severe activity in some countries where K dominated [8] [7] [9]. However, surveillance groups (and preliminary vaccine‑effectiveness networks) reported the 2024–25 vaccines still provided meaningful protection against H3N2 in prior seasons and public‑health sources emphasize that vaccination reduces hospitalizations even when antigenic drift occurs [10] [11]. Both lines — antigenic divergence raising concern and vaccines still offering partial protection — appear across the sources [3] [10].
6. What national agencies have done or communicated
National agencies and regional centers (ECDC, UKHSA, CDC reporting) are publishing surveillance updates and risk briefs highlighting the rise of subclade K and possible reduced recognition by vaccine‑induced antibodies; ECDC’s threat brief details the mutations and divergence from the A/Croatia vaccine strain [3]. Media accounts cite national hospital impacts and urge vaccination; sources show public health bodies are monitoring and advising but none in the provided material announced a formal mid‑season swap of the already recommended NH 2025–26 H3N2 vaccine virus [4] [12] [3].
7. What’s not in the reporting / important limitations
Available sources do not mention any WHO order or national regulatory approval that retroactively replaced the NH 2025–26 A/Croatia/10136RV/2023-like H3N2 vaccine component after manufacturers began production (not found in current reporting). They also do not provide complete real‑world vaccine effectiveness against subclade K for the 2025–26 season yet — only early signals and antigenic lab data are reported [3] [10].
Bottom line: WHO published A/Croatia/10136RV/2023‑like as the H3N2 pick for Northern Hemisphere 2025–26 and later WHO Southern Hemisphere guidance altered H3N2 picks for 2026; surveillance since selection has documented a divergent H3N2 subclade K that raises mismatch concerns, but the reviewed sources show monitoring and communication rather than a mid‑season swap of the NH 2025–26 H3N2 vaccine strain [1] [2] [3].