Evaluation of influenza vaccine effectiveness in 2017-2018 influenza season based on community children cohort study

Abstract

Objective: To assess the effectiveness of influenza vaccine in children aged 6-72 months. Methods: The cohort study was conducted based on community child vaccination clinics in Yiwu and Yongkang counties of Zhejiang province. From October 2017 to December 2017, a total of 1 752 children aged 6-72 months were enrolled from 10 child vaccination clinics. The questionnaire survey was conducted after the written consents were obtained from the parents or legal guardians of the children. Then, a follow up was conducted for enrolle children until 30 April 2018, the influenza vaccination status and the number of influenza-like illness (ILI) cases, hospital visit due to ILI, self-medication due to ILI were observed and recorded every month. Vaccine effectiveness (VE) was estimated by using the generalized linear model (GLM) where dependent variables were the number of ILI cases, hospital visit and self-medication respectively. Results: Of the 1 752 children, 925 (52.80%) were boys and the median age was 30.00 months. The cumulative observation was 308 166 person days at the end of 2017-2018 season, with 5.27 ILI cases per 1 000 person days, 3.41 hospital visit due to ILI per 1 000 person days, 1.45 self-medication due to ILI per 1 000 person days. Of the 1 752 children, 643 received the influenza vaccination in 2017-2018 season. Compared with unvaccinated children, the VE was 23.5% against ILI case number (95%CI: 15.1%-31.1%), 19.3% against hospital visit due to ILI (95%CI: 8.2%-29.1%) and 25.8% against self-medication due to ILI (95%CI: 9.3%- 39.3%). Modeling splitting 643 children with 2017-2018 vaccination into those before and after vaccination, the influenza VE was 31.9% against ILI case number (95%CI: 12.7%-46.9%), 32.6% against hospital visit due to ILI (95%CI: 8.6%-50.3%) and 44.3% against self-medication due to ILI (95%CI: 11.9%-64.8%) in children aged 36-72 months. However, the children aged 6-35 months showed no significant VEs. For the VE analysis in children with different vaccination status, the VEs were significant if they received vaccination in both 2016-2017 season and 2017-2018 season or only in 2017-2018 seasons. The VE was not demonstrated among the children who were immunized only in 2016-2017 season. Conclusion: Influenza vaccination is moderate effective in preventing the incidence of ILI and hospital visit and self-medication in children in influenza season, the protection effect in children aged 36-72 months is better than that in children aged 6-35 months.