Objective  To analyze protective effect of seasonal influenza vaccination among primary and junior middle school students during influenza outbreak in schools during 2022 – 2023 influenza for developing influenza vaccination strategies.

  Methods  A retrospective cohort study was conducted among 4 087 students in 64 classes of 5 preschool/primary/junior middle schools with influenza outbreaks (at least two cases diagnosed in a class) during February – April 2023 in Sichuan province. The parents of the students were asked to answer an online questionnaire about the students′ information on demographics, influenza situation, and the status of influenza vaccination. The cases′ laboratory diagnosis and the students′ detailed information on seasonal vaccination were further checked in the National Infectious Disease Surveillance Information System and the Sichuan Province Planned Immunization Information Management System using Identification Card Numbers of the students. Multivariate logistic regression was adopted to calculate adjusted relative risk (RR) of influenza of the influenza vaccinees for the estimation of adjusted vaccine effectiveness (VE_adj) of influenza vaccination with following formula: VE_adj = (1 – RR_adj) × 100%.

  Results  Among all the students surveyed, 1 352 (33.1%) had influenza onset during the outbreak period from April to June 2023, including 292 laboratory confirmed, 659 clinically diagnosed, and 401 suspected cases. There were 19.0% of students had receiving seasonal influenza vaccination, the VE_adj (95% confidence interval 95%CI) of flu vaccination 26.0% (0.6% – 44.9%) for clinically diagnosed and laboratory confirmed cases; stratified analyses showed that for all the vaccinees with the interval time of within 14 – 120 days and within 121 – 180 days between the vaccination and influenza onset, the VE_adj (95%CI) were 78.3% (59.8% – 88.3%) and 58.6% (35.6% – 73.4%); for the vaccinees with the same two interval periods but receiving quadrivalent flu vaccine, the VE_adj (95%CI) were 81.4% (63.8% – 90.4%) and 62.3% (40.7% – 76.0%) and the VE_adj (95%CI) were 73.4% (12.1% – 92.0%) and 87.1% (74.7% – 93.4%) for the vacinees with confirmed influenza A(H1N1) infection and the same two interval periods mentioned above, respectively. No significant protective effect of influenza vaccine for the vaccinees with the interval time of more than 180 days time intervals >180 d; the VE_adj (95%CI) for vaccination time intervals between 14, 120 d and (120, 180 d for between the vaccination and influenza onset.

  Conclusion  Influenza vaccination is of certain protective effect on preventing influenza infection among preschool/primary/junior middle school students, and the protective effect is obvious within 6 months after the seasonal vaccination. The results suggest influenza vaccination should be promoted among primary and junior middle school students to prevent influenza outbreak in schools.