Influenza vaccine effectiveness in primary and secondary school students: an ILI surveillance-based test-negative design case-control study in a district of Shenzhen city during 2021 – 2022 influenza season

  Objective  To evaluate the vaccine effectiveness (VE) of inactivated trivalent influenza vaccine in primary and secondary school students.

  Methods  Respiratory specimens were detected with PCR for influenza virus nucleic acid among 286 primary and secondary school students (6 – 15 years old) diagnosed as influenza-like illness (ILI) cases at all fever clinics in Nanshan district of Shenzhen city during an influenza season (from November 2021 to May 2022) and the ILI cases’ information on demography and influenza vaccination were collected simultaneously. The ILI cases positive for viral nucleic acid were assigned into a case group and those negative into a control group. Influenza vaccination ratio and odds ratio OR of viral nucleic acid positivity for the two groups were analyzed and compared to estimate the VE of influenza vaccination.

  Results  Of the 286 ILI cases, 68 (23.78%) were positive for influenza nucleic acid (all belonging to B strain) and 218 (76.22%) were negative. The ratio of seasonal influenza vaccination was 5.88% (4/64) for the cases and 15.14% (33/216) for the controls, and the OR of being virus nucleic acid positive was 0.3886 (95% confidence interval CI: 0.1528 – 0.9880) for the vaccinated ILI cases. The estimated VE was 61.14% (95%CI: 1.20% – 84.72%) for seasonal influenza vaccination.

  Conclusion  Seasonal influenza vaccination can protect primary and secondary school students from influenza virus infection. The result suggests that seasonal influenza vaccination needs to be promoted among school age children.