What is the measured effectiveness of the 2025-2026 H3N2 vaccine in different age groups?
Executive summary
Early, preprint and government reporting indicates the 2025–26 influenza vaccine is providing strong protection against serious H3N2 illness in children but more modest protection in adults: UKHSA and UK government summaries report 70–75% effectiveness at preventing hospital attendance in children aged 2–17 and about 30–40% effectiveness in adults [1] [2]. European and WHO reports warn a drifted H3N2 subclade K is circulating widely and could affect overall VE; laboratory antigenic testing and surveillance data are cited in those reports [3] [4].
1. Hard numbers: what the early VE estimates say
The UK Health Security Agency’s early, preprint VE analysis — highlighted by UK government communications and news outlets — finds the 2025–26 vaccine reduces the chance of hospital attendance by roughly 70–75% in children aged 2–17 and by about 30–40% in adults [1] [2] [5]. These figures are described as “early” and derived from emergency department and hospital-attendance outcomes rather than final, peer‑reviewed national VE estimates [1] [5].
2. Why children look better protected than adults
UKHSA’s early data and government messaging emphasize that protection in children is “particularly encouraging” despite dominance of the drifted subclade K; those messages frame high VE in children as a robust finding from the preprint analysis [2]. Available sources do not mention detailed mechanistic explanations in those briefs, but reporting and agency summaries link higher pediatric VE to observed hospital-attendance reductions in that age band [2] [5].
3. The circulating virus: subclade K and the vaccine match problem
European Centre for Disease Prevention and Control (ECDC) and WHO assessments document the rapid rise of a drifted A(H3N2) subclade K with multiple haemagglutinin substitutions relative to the 2025–26 vaccine reference strain; ECDC warns the subclade shows substantial divergence from the vaccine virus [3]. WHO serology papers describe human and ferret assay results and note ongoing diversification of clade 2a.3a.1 viruses, which underpins concern about potential reductions in population-level protection [4].
4. Reconciling lab data and real-world VE — competing signals
Laboratory antigenic testing shows some reductions in recognition of recent H3N2 viruses by vaccine strains, prompting caution that clinical VE could be affected [4] [6]. Yet the UKHSA preprint reports meaningful real‑world protection against hospital attendance in children and moderate adult protection, illustrating a common situation: antigenic drift can reduce—but does not necessarily eliminate—clinical protection, especially against severe outcomes [1] [2] [6].
5. Wider surveillance and geography matter
Reports emphasize that the subclade K proportion varies by region — ECDC notes near‑half prevalence in EU/EEA submissions and other sources document its spread to multiple countries — so VE measured in the UK may not directly translate to other countries where different subclades predominate or where vaccine formulations (egg‑ vs cell‑propagated) differ [3] [4] [7]. CDC materials reiterate national vaccination recommendations but available CDC pages in the search set do not yet provide detailed US-specific VE estimates for H3N2 in 2025–26 [8].
6. Limits, caveats and unanswered questions
The primary VE numbers come from a preprint and early surveillance analyses; UKHSA’s findings are described as interim and not yet peer‑reviewed [1] [2]. Laboratory and genomic reports show a drifted subclade that could change VE as the season unfolds, and regional differences in circulating viruses and vaccine production methods can alter effectiveness locally [3] [4]. Available sources do not provide finalized, peer‑reviewed age‑stratified VE estimates across all countries or longer-term effectiveness [1].
7. Practical takeaway for clinicians and the public
Public‑facing agencies and professional outlets continue to advise vaccination as the best available tool to reduce severe outcomes; the UK and U.S. guidance reiterates vaccination for everyone ≥6 months and notes substantial vaccine supply and distribution plans [8] [2] [9]. Given the early evidence — strong pediatric protection and modest adult protection against hospital attendance — vaccination is positioned as particularly important to prevent severe illness even in a partially mismatched season [2] [5].
Sources cited above are the UKHSA preprint and related UK government release [1] [2], contemporaneous reporting and analysis [5] [7], WHO serology and vaccine‑composition documents [4], ECDC risk brief on subclade K [3], and U.S. CDC seasonal guidance [8].