Which influenza strains are driving hospitalizations in 2025–2026 and how do they affect vaccine effectiveness?
Executive summary
H3N2 — specifically a mutated H3N2 subclade called “subclade K” — is driving early outbreaks abroad and is expected to be a major driver of hospitalizations in 2025–2026 if it spreads widely in the U.S.; public reporting shows early U.K., Canadian and Japanese surges and that subclade K already appears in multiple countries [1] [2] [3]. U.S. vaccines for 2025–26 are trivalent (H1N1, H3N2 and influenza B), based on H3N2 reference strains chosen before subclade K emerged, so experts say vaccine effectiveness against infection may be reduced but should still protect substantially against severe outcomes and hospitalizations [4] [5] [6].
1. New H3N2 subclade K: an emerging driver of hospital admissions abroad
Public health agencies in Canada, the U.K. and Japan have reported early waves of illness tied to a mutated H3N2 subclade known as “K,” and journalistic reporting describes those waves as already producing hospital admissions and warnings from local officials [1] [2] [3]. Multiple outlets report that subclade K acquired several mutations after vaccine reference strains were selected over the summer, which helps explain concern about how it could change the season’s hospitalization picture if it becomes dominant in the U.S. [2] [5].
2. What’s in the U.S. 2025–26 shot and why that matters
The FDA and federal partners recommended a trivalent vaccine for 2025–26 that contains two influenza A subtypes (H1N1 and H3N2) and one influenza B lineage; the H3N2 component selected for manufacturing is an A/Croatia/10136RV/2023-like virus for egg-based vaccines [4]. Those strain choices were finalized before the summer emergence and spread of subclade K; that timing is the proximate reason scientists flag a potential mismatch between the vaccine H3N2 component and subclade K [5] [4].
3. Vaccine effectiveness: reduced infection protection but retained defense against severity
Experts interviewed in coverage and early analyses from health agencies indicate the vaccine may be less effective at preventing infection with subclade K than with the reference H3N2 strains, but multiple sources stress that even a imperfect match generally provides meaningful protection against severe outcomes and hospitalizations [6] [7] [8]. Real-world early estimates from other countries show mixed performance — for example, UK preprint results cited in reporting found vaccine effectiveness against ED visits and hospital admission ranged from about 32–39% in adults and higher (72–75%) in children in settings where subclade K dominated early — figures that fit typical seasonal ranges and suggest retained benefit against severe disease [8].
4. Context from last season: why co-circulating A strains matter
The U.S. 2024–25 season was unusually severe with co-circulation of H1N1 and H3N2 and record-high hospitalization rates in some surveillance metrics; that season’s experience is the reference point for current worry because H3N2-dominant seasons historically hit older adults harder and often show lower vaccine effectiveness overall [9] [10] [11]. CDC surveillance shows massive prior-season hospitalization counts used to frame risk — for example, FluSurv-NET reported tens of thousands of laboratory-confirmed influenza hospitalizations last season, underscoring why a poorly matched H3N2 could matter [12] [10].
5. Uncertainties and limitations in current reporting
Available sources show subclade K’s spread internationally and note it has been detected in multiple U.S. states, but U.S. national dominance is not yet established in the cited material; weekly U.S. surveillance in November 2025 still showed low but rising hospitalization rates nationally and regionally with reporting delays noted [13] [14]. The exact impact of subclade K on severity compared with other strains is unclear — some experts caution it may not be intrinsically more virulent but could drive hospitalizations simply by producing more infections [6]. Available sources do not mention definitive U.S. dominance of subclade K or final, peer‑reviewed vaccine effectiveness estimates for the U.S. 2025–26 season.
6. Practical takeaway for clinicians and the public
Federal agencies recommend annual vaccination for everyone 6 months and older; the FDA and CDC maintain the 2025–26 trivalent vaccine supply and recommendations while stressing that vaccination reduces severe illness and hospitalizations even when strain match is imperfect [15] [4]. Multiple experts and public-health reporters emphasize that an imperfect H3N2 match is a reason to prioritize vaccination and antivirals for high-risk patients, because reduced protection against infection does not equate to no protection against severe outcomes [7] [8].
Sources cited above are drawn from CDC reporting and federal vaccine recommendations [12] [13] [14] [4] [15] and contemporary journalism and expert commentary documenting subclade K’s international spread and early vaccine‑effectiveness signals [5] [1] [2] [6] [7] [8].