Eurosurv. Modest protection from vaccination against influenza A(H3N2) subclade K, Beijing, China, 2025/26 season

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• Ying Shen[SUP]1,2,*[/SUP] , Daitao Zhang[SUP]1,2,3,*[/SUP] , Zhaomin Feng[SUP]1,2,3,*[/SUP] , Chunna Ma[SUP]1[/SUP] , Weixian Shi[SUP]1[/SUP] , Wei Duan[SUP]1[/SUP] , Jia Li[SUP]1[/SUP] , Lu Zhang[SUP]1[/SUP] , Dan Wu[SUP]1[/SUP] , Jiaojiao Zhang[SUP]1[/SUP] , Jiaxin Ma[SUP]1[/SUP] , Yingying Wang[SUP]1[/SUP] , Xiaodi Hu[SUP]1[/SUP] , Shuning Yan[SUP]1[/SUP] , Yuanzhi Di[SUP]1[/SUP] , Jiachen Zhao[SUP]1[/SUP] , Hui Xu[SUP]1[/SUP] , Guilan Lu[SUP]1[/SUP] , Yimeng Liu[SUP]1[/SUP] , Weijia Zhang[SUP]1[/SUP] , Quanyi Wang[SUP]1,2,3,**[/SUP] , Peng Yang[SUP]1,2,3,**[/SUP]

​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.

https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.7.2600096