Objective To evaluate the vaccine effectiveness (VE) of influenza vaccine (InfV) in pregnant women on their infants in Gansu province.

Methods A total of 1 229 978 infants born to pregnant women in Gansu province from January 1, 2018 to December 31, 2023 were included in this study. Data were collected from the electronic medical record system, birth registration system, and immunization program system of Gansu province. A retrospective cohort of pregnant women who received InfV during pregnancy was constructed by linking the databases of each system through ID number. Information on laboratory-confirmed influenza cases in infants was collected through the electronic medical record system. Robust Poisson regression analysis was used to compare the adjusted risk ratio (RR), VE, and 95%confidence interval (CI) of influenza in infants born to pregnant women who received InfV during pregnancy and those who did not from 2018 to 2023.

Results The incidence of influenza in infants born to 1 210 pregnant women who received InfV during pregnancy was 4.71% (57/1 210), which was significantly lower than 6.13% in infants born to 1 228 768 pregnant women who did not receive InfV during pregnancy (χ²=4, P<0.05). The adjusted RR was 0.78 (95%CI: 0.29–0.99), and the VE was 22.13% (95%CI: 0.79–40.30).

Conclusions InfV vaccination during pregnancy provides some protection against influenza in infants under 6 months of age.