Moderate protection from vaccination against influenza A(H3N2) subclade K in Beijing, China, September to December 2025.
Ying Shen, Daitao Zhang, Zhaomin Feng, Chunna Ma, Weixian Shi, Wei Duan, Jia Li, Lu Zhang, Dan Wu, Jiaojiao Zhang, Jiaxin Ma, Yingying Wang, Xiaodi Hu, Shuning Yan, Yuanzhi Di
Abstract
Open AccessDuring 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 September and December 2025. Among 9,579 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 41.3% (95% CI: 29.2 to 51.3), indicating moderate protection during this subclade K-dominated season.