Influenza Vaccine Effectiveness in School Outbreaks During a A(H3N2) Subclade K (J.2.4.1)-Dominated Season in Beijing, China, 2025–26

Abstract:

"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."

Shen, Y., Ma, C., Zhang, D., Shi, W., Duan, W., Li, J., Zhang, L., Wu, D., Zhang, J., Ma, J., Wang, Y., Hu, X., Yan, S., Di, Y., Zhao, J., Xu, H., Lu, G., Liu, Y., Wang, Q. and Yang, P. (2026). Influenza Vaccine Effectiveness in School Outbreaks During a A(H3N2) Subclade K (J.2.4.1)-Dominated Season in Beijing, China, 2025–26. International Journal of Infectious Diseases, [online] p.108777. doi:https://doi.org/10.1016/j.ijid.2026.108777