Credit ACIP/CDC
#18,943
Last weekend, in Increasing Concerns Over A `Drifted' H3N2 Virus This Flu Season, we looked at early warnings of a major `drift' in this year's H3N2 virus. Since then we've seen a growing number of media headlines like:
Flu season "Worst in over 30 years" says physician
Warnings rise for U.S. as severe flu strain causes outbreaks in Canada, U.K.
Mutant virus strain 'could overwhelm hospitals this winter' as health bosses sound alarm
Much of this media consternation comes comes following reports of early season flu surges in Asia (Japan, Taiwan, Guam, China, etc.) - which followed a rough Australian flu season (see Dr. Ian Mackay's A Flunami in July) - and a very recent preprint by Dr. Danuta Skowronski et al., which characterized recent changes in the H3N2 virus that and warned:
This `. . . . vaccine mismatched A(H3N2) variant ' is `. . . projected to predominate during the northern hemisphere’s 2025–2026 influenza season.'All of which has some people wondering: Is it worth getting the flu vaccine this year?
While my own take is ` even a little protection is better than none' (full disclosure: I get the flu vax every year), we get a more scientific take from the UK's Health Security Agency, which published a (very) early season analysis of this year's vaccine effectiveness (VE) against this year's drifted H3N2 virus.
New data published today as a pre-print by the UK Health Security Agency (UKHSA) shows the 2025/26 vaccine is currently 70 to 75% effective at preventing hospital attendance in children aged 2 to 17 years and 30 to 40% effective in adults.
The early findings confirm the vaccine provides important protection even though a drifted influenza A(H3N2) strain (also now known as subclade K) currently dominates cases in England, so the protection levels observed in children are particularly encouraging this year. Flu vaccine effectiveness varies from season to season but is typically between 30 to 60%, with higher vaccine effectiveness typically seen in younger age groups.
High effectiveness in younger age groups may also provide indirect protection to other groups through reduced transmission.
The high (70 to 75%) VE in children is encouraging, but most children in the UK receive the LAIV (nasal) vaccine; not the IIV (shot) which adults typically receive, and their vaccinations were quite recent.
I've reproduced some excerpts from the preprint below, but you'll want to read it in its entirety. I'll have a bit more after the break.
Freja CM Kirsebom1, Catherine Thompson2, Tiina Talts2, Beatrix Kele2, Heather J Whitaker3, Nurin Abdul Aziz1, Christopher Rawlinson1, Rebecca E Green1, Catherine Quinot1, Nicholas Gardner1,Elizabeth Waller1, Alex Allen1, Conall H Watson1,4, Suzanna LR McDonald1, Maria Zambon2,Richard Pebody4,5, Mary Ramsay6,7, Katja Hoschler2, Anika Singanayagam*2,4, Jamie Lopez Bernal*1,4*AbstractInfluenza A(H3N2) subclade K (J.2.4.1) has dominated the start of the 2025 to 2026 influenza season in England. We found reduced reactivity of subclade K with post-infection ferret antisera raised against the Northern Hemisphere 2025 to 2026 vaccine strains, aligning with World Health Organization reports.
Nevertheless, vaccine effectiveness against hospital attendance and admission in the early season currently remains within a typical range at 70 to 75% in children and 30 to 40% in adults. Our data indicates that vaccination remains an effective preventative tool against circulating influenza A(H3N2).
The end of the Southern Hemisphere 2025 influenza season and the start of the Northern Hemisphere (NH) 2025 to 2026 season has seen rapid growth in activity of influenza A(H3N2)subclade K (formerly referred to as subclade J.2.4.1) which is projected to dominate among H3N2 viruses during the 2025 to 2026 season (1 to 3).
The K subclade marks a notable evolution in influenza A(H3N2) viruses since the NH 2025 to 2026 candidate vaccine strains were selected(based on the J.2 subclade) and are characterised by T135K, K189R with 7 additional mutations(HA1: K2N, S144N, N158D, I160K, Q173R, ) (2 to 4).
Early analysis from the World Health Organization (WHO) influenza vaccine composition meeting (VCM) in September 2025 suggest low reactivity of these viruses with post-infection ferret antisera raised against the NH vaccine strains (5). It is not yet clear how 2025 to 2026 vaccine effectiveness (VE) against clinical disease may be affected by these observations.
Here, we report genetic and antigenic characterisation of H3N2 viruses and real-world VE against emergency department (ED)attendance and hospital admission with influenza A in England.
Epidemiological context
Influenza activity in England began unusually early in the 2025 to 2026 season, with a rise in influenza like illness emergency department attendances and influenza test positivity amongteenagers and young adults followed by increases in younger children (6).
Most all-age influenza indicators, including influenza hospitalisations and intensive care unit admissions went above baseline levels from week 43 2025, the earliest inter-pandemic start to the season in England since 2003 to 2004 (6). Almost all cases (98%) since week 40 have been influenza A and, where subtyping was available for these, 84% were A(H3N2) (Supplementary Figure 1). Early starts to the 2025 to 2026 season have also been observed elsewhere in the NH (7, 8)
(SNIP)
Influenza A(H3N2) genetic characterisationGenetic characterisation by whole genome sequencing of viruses collected from primary and secondary care in England has shown predominance of antigenically drifted A(H3N2) viruses belonging to subclade K since week 35 2025; 156 of 179 (87%) belong to this novel subclade (Table 1 and Supplementary Figure 2). Some J.2 and J.2.4 viruses continue to be detected. The distribution of all genetically characterised influenza A detections since week 20 2025 is described in Supplementary Figure 2a.
(SNIP)
Overall, our results are encouraging, though it is important to note that these observations are from a period very soon after vaccination when there has been no waning in effectiveness. Recent end of season analyses highlighted waning in effectiveness in adults over the course of an influenza season (Whitaker et al., 2025, In press, DOI: 10.1111/irv.70194) and it will be important to continue to monitor duration of protection this year.
To date, the burden of influenza has been greatest in children and young adults in England. The lower VE results seen in adults, when compared to children, is similar to previous seasons but does not exclude a larger reduction in protection in older adults as the season progresses.
Alternatively, the reason for the lower VE results in adults than children may be explained by higher pre-existing background immunity against the circulating strains, in which cases the vaccine would offer less additional benefit. Seroprevalence analyses will be needed to confirm this.
(Continue . . . )Conclusion
Despite the emergence of a drifted influenza A(H3N2) strain driving an unusually early 2025 influenza season in England and some other countries in the NH, our early estimates provide reassurance that the 2025 to 2026 NH enhanced vaccines provide important protection in children and adults in the early period post-vaccination. The high VE in children, strengthens the case for optimising vaccine uptake in this group, where we may also see indirect protection of other age cohorts (23).
When I got my flu shot last month I already had a strong inkling that the H3N2 component was going to be `suboptimal', as the WHO had already changed the formulations for next year's Southern Hemisphere flu shot.
But the flu shot is just part of my `flu prevention' routine each year. Something I take more seriously as my age, and comorbidities, rise.
I continue to wear a mask in crowded indoor venues, I carry (and use) copious amounts of hand sanitizer, and I purposely avoid high risk encounters (i.e., very large crowds).
While none of this guarantees I'll go unscathed this year, I've had pretty good luck over the years. My last `flu' was in 2009 (after a cross-country plane trip), and I've only contracted COVID once, before the first vaccine was released.
Even if this year's vaccine turns out to be a total bust with H3N2, H1N1 will continue to circulate at low levels, and influenza B could loom large later in the season.
Flu season is, afterall, nothing if not unpredictable.
