Background
Influenza A subclade K viruses caused high infection rates in the 2025/2026 Northern Hemisphere season, raising concerns about antigenic drift and reduced vaccine effectiveness.
Methods
We measured antibody responses in matched human pre- and post-vaccination sera, selected from two observational cohort studies of adults, against both a vaccine-like as well as subclade K isolates.
Findings
Pre-existing immunity to subclade K variants was noted with seasonal influenza vaccination further boosting titres two-fold against subclade K and three-fold against the vaccine-like strain, consistent with limited antigenic divergence between subclade K isolates and the vaccine. These findings contrast with ferret-based predictions of marked antigenic drift and align with the observed vaccine effectiveness in adults.
Interpretation
Our results underscore the importance of incorporating human serologic data in influenza surveillance to better inform vaccine strain selection and anticipate vaccine performance in immunologically experienced populations.