How does the 2025-2026 H3N2 vaccine effectiveness compare to previous seasons?
Executive summary
Early 2025–26 data show the Northern Hemisphere vaccine still protects against current H3N2 activity: UK analyses estimate 70–75% effectiveness at preventing hospital attendance in children (2–17 years) and 30–40% in adults despite the emergence of an H3N2 “subclade K” that is antigenically drifted from the vaccine reference strain [1] [2] [3]. Laboratory and serology tests show reduced antibody recognition of some circulating H3N2 viruses versus the vaccine strain, consistent with WHO and national lab findings [4] [5] [6].
1. What the early numbers say — vaccine still preventing severe outcomes
Real‑world, early-season analyses from the UK Health Security Agency report that vaccinated children are about 70–75% less likely to attend or be admitted to hospital with influenza this season, and vaccinated adults about 30–40% less likely to do so [2] [1]. Independent reporting and international summaries have repeated these figures and framed them as “within typical ranges” for seasonal flu vaccine protection against severe illness even when a drifted H3N2 variant is circulating [7] [3].
2. Why lab tests look worrying — antigenic drift and ferret sera
Laboratory assays, including ferret antisera and human serology, show reduced reactivity between antibodies raised against the vaccine reference H3N2 strain and some recent viruses, notably subclade K and other J.2 descendants; in some datasets a sizable fraction of H3N2 isolates showed 2‑ to 4‑fold reduced titres [4] [5] [6]. Reduced recognition in these assays signals antigenic drift — it does not directly equal clinical failure, but it raises the risk of lower vaccine effectiveness against infection and mild disease [4] [6].
3. How this season compares to prior H3N2 years — consistent pattern of lower VE
Public health scientists note that H3N2‑dominant seasons historically produce lower vaccine effectiveness and greater severity in older adults compared with H1N1‑dominant seasons; experts cited in reporting call H3N2 “the nastier” subtype and expect lower VE during H3N2 waves [3] [8]. That historical pattern helps explain why a drifted H3N2 variant prompts heightened concern even when early VE for severe outcomes remains acceptable [3] [8].
4. Geographic and genomic context — subclade K is spreading but not uniform
Genomic surveillance through mid‑late 2025 shows subclade K emerging and making up a substantial share of H3N2 sequences in some regions (for example, ~33% of global H3N2 GISAID deposits and ~47% in the EU/EEA in one report), but circulation and subclade frequencies vary by country and over time [8]. WHO summaries and national reports emphasize ongoing diversification within the J.2 family of H3N2 viruses and that not all recent viruses are equally poorly recognized by vaccine‑raised antibodies [4] [5].
5. What “mismatch” means in practice — lab signal vs. clinical protection
Authors of the WHO and national reports stress a distinction: antigenic assays show reduced recognition, but real‑world VE against severe outcomes can remain “meaningful” because vaccines stimulate broader immune responses and can still reduce hospitalizations even with drift [4] [1] [6]. Multiple outlets synthesize this: ferret and serology tests point to immune escape potential, yet early observational VE studies show substantial protection against severe disease in children and modest protection in adults [7] [1] [2].
6. Limitations and data gaps to watch
Current VE estimates are early, come from specific settings (principally England) and measure hospital attendance/admission rather than all infections; they may not generalize globally or to mild disease [7] [9] [2]. Surveillance gaps, variable sequencing coverage, and evolving viral genetics mean effectiveness estimates could change as the season progresses [8] [4]. Available sources do not mention long‑term VE estimates through the full season or comprehensive age‑stratified global VE analyses.
7. What experts recommend and why it matters
Reporting and public health messaging emphasize vaccination remains the best defense against severe influenza this season even with antigenic drift, and encourage continued surveillance to detect shifts in VE or disease burden [1] [3] [8]. Scientific literature on vaccine strain selection also highlights that later or more reproducible selection and better genomic surveillance could reduce future mismatches — a structural point raised by researchers reviewing A/H3N2 vaccine selection processes [10].
Takeaway: early 2025–26 evidence shows a laboratory signal of antigenic drift in H3N2 (subclade K) but real‑world early VE against hospital attendance remains substantial in children (70–75%) and modest in adults (30–40%) in UK data; these figures align with public health agencies’ messaging while surveillance and wider, season‑long VE studies are still pending [4] [1] [2] [3].