tetano
Editor, Senior Moderator
Int J Infect Dis
. 2026 May 9:108777.
doi: 10.1016/j.ijid.2026.108777. Online ahead of print.
Influenza Vaccine Effectiveness in School Outbreaks During a A(H3N2) Subclade K (J.2.4.1)-Dominated Season in Beijing, China, 2025-26
Ying Shen[SUP] 1 [/SUP], Chunna Ma[SUP] 2 [/SUP], Daitao Zhang[SUP] 1 [/SUP], Weixian Shi[SUP] 2 [/SUP], Wei Duan[SUP] 2 [/SUP], Jia Li[SUP] 2 [/SUP], Lu Zhang[SUP] 2 [/SUP], Dan Wu[SUP] 2 [/SUP], Jiaojiao Zhang[SUP] 2 [/SUP], Jiaxin Ma[SUP] 2 [/SUP], Yingying Wang[SUP] 2 [/SUP], Xiaodi Hu[SUP] 2 [/SUP], Shuning Yan[SUP] 2 [/SUP], Yuanzhi Di[SUP] 2 [/SUP], Jiachen Zhao[SUP] 2 [/SUP], Hui Xu[SUP] 2 [/SUP], Guilan Lu[SUP] 2 [/SUP], Yimeng Liu[SUP] 2 [/SUP], Quanyi Wang[SUP] 3 [/SUP], Peng Yang[SUP] 3 [/SUP], Zhaomin Feng[SUP] 4 [/SUP]
Affiliations
Background: Schools are high-risk settings for influenza transmission. During the 2025-26 Northern Hemisphere season, influenza A(H3N2) subclade K (J.2.4.1) rapidly emerged and became predominant, but vaccine effectiveness (VE) in school-associated outbreaks remains unclear.
Methods: We conducted a field investigation of 93 school-associated febrile outbreaks in Beijing, China, from September to December 2025. Cases were febrile students with laboratory-confirmed influenza, and afebrile classmates served as controls. VE against laboratory-confirmed symptomatic infection was estimated using mixed-effects logistic regression, adjusting for age, sex, school type, and outbreak-level clustering.
Results: A total of 3,797 students were included, of whom 788 were laboratory-confirmed febrile cases and 3,009 were afebrile classmates. Primary schools accounted for 74.2% of outbreaks. Overall vaccination rate was 54.6%, with 50.6% among cases and 56.1% among controls. Unadjusted VE was 19.6% (95% CI: 5.9-31.3). After adjustment, VE against laboratory-confirmed influenza was 30.0% (95% CI: 17.3-40.7). Sensitivity analyses yielded consistent results.
Conclusions: Influenza vaccination provided modest protection against symptomatic A(H3N2) infection in school-aged children, even under conditions of high-intensity exposure and antigenic drift. These findings underscore the value of vaccination in outbreak-prone school settings and highlight the importance of ongoing vaccine strain optimization and outbreak-based VE surveillance.
Keywords: H3N2; influenza; school outbreaks; vaccine effectiveness.
. 2026 May 9:108777.
doi: 10.1016/j.ijid.2026.108777. Online ahead of print.
Influenza Vaccine Effectiveness in School Outbreaks During a A(H3N2) Subclade K (J.2.4.1)-Dominated Season in Beijing, China, 2025-26
Ying Shen[SUP] 1 [/SUP], Chunna Ma[SUP] 2 [/SUP], Daitao Zhang[SUP] 1 [/SUP], Weixian Shi[SUP] 2 [/SUP], Wei Duan[SUP] 2 [/SUP], Jia Li[SUP] 2 [/SUP], Lu Zhang[SUP] 2 [/SUP], Dan Wu[SUP] 2 [/SUP], Jiaojiao Zhang[SUP] 2 [/SUP], Jiaxin Ma[SUP] 2 [/SUP], Yingying Wang[SUP] 2 [/SUP], Xiaodi Hu[SUP] 2 [/SUP], Shuning Yan[SUP] 2 [/SUP], Yuanzhi Di[SUP] 2 [/SUP], Jiachen Zhao[SUP] 2 [/SUP], Hui Xu[SUP] 2 [/SUP], Guilan Lu[SUP] 2 [/SUP], Yimeng Liu[SUP] 2 [/SUP], Quanyi Wang[SUP] 3 [/SUP], Peng Yang[SUP] 3 [/SUP], Zhaomin Feng[SUP] 4 [/SUP]
Affiliations
- PMID: 42114651
- DOI: 10.1016/j.ijid.2026.108777
Background: Schools are high-risk settings for influenza transmission. During the 2025-26 Northern Hemisphere season, influenza A(H3N2) subclade K (J.2.4.1) rapidly emerged and became predominant, but vaccine effectiveness (VE) in school-associated outbreaks remains unclear.
Methods: We conducted a field investigation of 93 school-associated febrile outbreaks in Beijing, China, from September to December 2025. Cases were febrile students with laboratory-confirmed influenza, and afebrile classmates served as controls. VE against laboratory-confirmed symptomatic infection was estimated using mixed-effects logistic regression, adjusting for age, sex, school type, and outbreak-level clustering.
Results: A total of 3,797 students were included, of whom 788 were laboratory-confirmed febrile cases and 3,009 were afebrile classmates. Primary schools accounted for 74.2% of outbreaks. Overall vaccination rate was 54.6%, with 50.6% among cases and 56.1% among controls. Unadjusted VE was 19.6% (95% CI: 5.9-31.3). After adjustment, VE against laboratory-confirmed influenza was 30.0% (95% CI: 17.3-40.7). Sensitivity analyses yielded consistent results.
Conclusions: Influenza vaccination provided modest protection against symptomatic A(H3N2) infection in school-aged children, even under conditions of high-intensity exposure and antigenic drift. These findings underscore the value of vaccination in outbreak-prone school settings and highlight the importance of ongoing vaccine strain optimization and outbreak-based VE surveillance.
Keywords: H3N2; influenza; school outbreaks; vaccine effectiveness.