Objective  To estimate vaccine effectiveness (VE) of seasonal influenza vaccine inoculation during influenza seasons among children aged 6 to 72 months using screening method and to provide references for developing and improving the seasonal influenza vaccination in China.

  Methods  We extracted the data on registered 6 – 72 months old influenza cases during flu seasons of 2016 – 2017 and 2017 – 2018 (from October in a year to April of the next year) in Yongkang and Yiwu city of Zhejiang province from China Influenza Surveillance Network. The information on influenza vaccine inoculation of the flu cases and those of all the children aged 6 – 72 months were checked in National Immunization Planning Information Management System to calculate the proportion of cases vaccinated (PCV) and the proportion of population vaccinated (PPV). A crude vaccine effectiveness (VE) value was estimated by PCV and PPV using the formula based on screening method. Then, the adjusted VE and its 95% confidence interval (CI) were estimated using generalized linear models (GLM).

  Results  Of the 458 influenza cases registered during the two flu seasons, 403 were included in the analysis after excluding 53 with unknown vaccination status and 2 with an interval less than 14 days between the vaccination and the flu incident. For the flu season of 2016 – 2017, totally 277 influenza cases were registered; the PCV and PPV were 6.1% and 7.7%; the crude VE was 22.1%; the adjusted VE (95% CI) was 30.6% (– 198.8% – 83.9%). For the flu season of 2017 – 2018, totally 126 influenza cases were registered; the PCV and PPV were 4.0% and 9.2%; the crude VE was 58.9%; the adjusted VE (95% CI) was 55.1% (– 11.0% – 81.8%). During the 2016 – 2017 and 2017 – 2018 flu season for the children aged 6 – 35 months, the number of registered influenza cases were 135 and 57; the PCV were 3.4% and 5.3%; the PPV were 3.4% and 6.6%; the crude VE were 12.1% and 20.8%; the adjusted VE were 16.0% (95% CI: – 129.0% – 69.2%) and 11.2% (95% CI: – 190.4% – 72.9%); while, for the children aged 36 – 72 months, the number of registered influenza cases were 142 and 69; the PCV were 9.2% and 2.9%; the PPV were 11.1% and 11.3%; the crude VE were 18.9% and 76.6%; and the adjusted VE was 32.2% (95% CI: – 120.2% – 79.1%) and 74.4% (95% CI: – 5.3% – 93.8%), respectively.

  Conclusion  Among the 6 – 72 months old children in Yongkang and Yiwu city of Zhejiang province, the vaccine effectiveness of seasonal influenza vaccine during the 2017 – 2018 flu season is higher than that during 2016 – 2017 flu season and the vaccine effectiveness is higher among the children aged 36 – 72 months than that among those aged 6 – 35 months.