Which influenza A and B strains were recommended for the 2025-26 UK seasonal vaccine?
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Executive summary
For the UK 2025–26 seasonal influenza programme, manufacturers followed WHO advice to make trivalent vaccines containing two influenza A strains (H1N1 and H3N2) plus a single influenza B (Victoria) strain; the B/Yamagata component was removed because it has not been detected since 2020 [1] [2]. UK documents note the A(H3N2) vaccine strain used in the Northern Hemisphere 2025/26 formulation was A/Croatia/10136RV/2023‑like, and early surveillance shows circulating viruses were dominated by A(H3N2) subclade K, which is antigenically drifted from that vaccine strain [3] [4].
1. What the 2025–26 UK vaccine contained — the short answer
The UK’s flu programme used trivalent vaccines comprising two A strains (A(H1N1) and A(H3N2)) and one B strain (B Victoria lineage); the B/Yamagata lineage was omitted following WHO advice because of its apparent disappearance after March 2020 [1] [2]. Government guidance and UKHSA materials explicitly state “All vaccines used for the 2025 to 2026 season are trivalent” and that the trivalent formulation contains “2 A strains and a B Victoria strain” [5] [1].
2. Which specific A and B strains were referenced in official surveillance and guidance
Public-facing surveillance and guidance identify the Northern Hemisphere 2025/26 H3N2 vaccine reference as “A/Croatia/10136RV/2023‑like” [3]. Broadly, the programme continued to cover the common seasonal types — H1N1, H3N2 and influenza B — with influenza B specified as the Victoria lineage in line with WHO recommendations [1] [2].
3. Why vaccines became trivalent this season — policy and evidence
WHO and UK advisory bodies recommended moving back to trivalent formulations because no confirmed detections of B/Yamagata lineage viruses have been reported after March 2020, making inclusion of that component “no longer warranted” [2]. UK officials and JCVI-led decisions resulted in inactivated vaccines being reformulated for a 2‑A + 1‑B composition for 2025–26 [2] [1].
4. Antigenic mismatch and the rise of subclade K — what surveillance shows
Early genetic and antigenic characterisation across England found the start of 2025/26 dominated by A(H3N2) subclade K (J.2.4.1). Laboratory data indicate reduced reactivity of post‑infection ferret antisera against these subclade K viruses when tested against the Northern Hemisphere 2025/26 vaccine strains, aligning with WHO reports of antigenic drift [4] [1]. UKHSA antigenic testing also reported that characterised A(H3N2) viruses were not similar to the A/Croatia vaccine reference in the small set antigenically profiled to date [3].
5. Vaccine effectiveness in the real world — competing signals
Despite antigenic drift in H3N2, early real‑world VE estimates from UKHSA and peer‑reviewed analysis show meaningful protection against severe outcomes: vaccine effectiveness against hospital attendance/admission was about 72–75% in children and adolescents and around 32–39% in adults [6] [4]. Commentators and health officials interpret these data differently: public health bodies stress that vaccines still protect against severe disease and hospitalisation [7] [6], while some media coverage highlights the drift and calls for vigilance about reduced protection for some age groups [8] [9].
6. Limitations, outstanding questions and what sources don’t say
Official materials document which broad strains and vaccine references were used and report early VE and antigenic characterisation, but available sources do not mention a complete list of the exact H1N1 and B Victoria strain names as laid out in vaccine licence documents for every manufacturer (not found in current reporting). Long‑term implications of subclade K dominance for end‑of‑season VE and whether future WHO composition meetings will recommend a different H3N2 are still unfolding in subsequent surveillance and not settled in these documents [4].
7. What this means for people and policy
Policy consequences were immediate: the national programme urged eligible people to get vaccinated because vaccines still reduce severe illness and hospitalisation, even when drifted H3N2 viruses circulate [10] [6]. The move to trivalent vaccines reflects WHO and JCVI judgments about B/Yamagata’s disappearance, but surveillance data showing antigenic drift in H3N2 underline why ongoing sequencing, antigenic testing and timely WHO composition decisions remain critical [1] [4].