During the early 2025/26 influenza season, influenza A(H3N2) subclade K rapidly predominated in Beijing, China. Using a test-negative design, we estimated influenza vaccine effectiveness (VE) among influenza-like illness outpatients tested between weeks 40/2025 and 04/2026. Among 10,484 participants, sequencing of 316 randomly selected A(H3N2)-positive samples showed 84.8% were subclade K, and antigenic analysis of 65 viruses indicated antigenic divergence. Despite this, adjusted VE against laboratory-confirmed influenza was 23.5% (95% confidence interval: 11.7–33.7), indicating modest protection during this subclade K-dominated season.
Shen et al. (Thu,) studied this question.