Evaluation of the influenza vaccine effectiveness among children aged 6 to 72 months based on the test-negative case control study design

Abstract

Objective: To assess the effectiveness of seasonal influenza vaccine among children aged 6 to 72 months. Methods: The test-negative case control study was conducted based on available surveillance data which was from China Influenza Surveillance Information system (CSIS). From October 2016 to April 2017 and from October 2017 to April 2018,1 161 cases aged 6-72 months with influenza-like illness in Yongkang and Yiwu city, were selected as the study subjects, and the cases with influenza test-positive were selected as the case group (403 cases). Test-negative subjects were selected as control group (758 cases). The etiology and immunization data of the subjects were obtained from CSIS and Immune Information and Management System (IIMS) respectively. Vaccine effectiveness was estimated using multivariate logistic regression model,and the mixed effects of non-randomized control in TNCC study were equalized by using the propensity score (PS) method in the statistical analysis. Results: The age of the subjects was (2.44±1.60) years,and there were 681 boys (58.66%). The age of case group was (2.62±1.58) years, and there were 246 boys (61.04%). The case group was including of 237 cases (58.81%) of influenza A (H3N2), 92 cases (22.83%) of influenza A (H1N1) pmd09, 62 cases (15.38%) of influenza B(Victoria) lineage, 11 cases (2.73%) of influenza B (Yamagata) lineage and one case (0.25%) co-infection of influenza [A(H(3)N(2))+B (Victoria)]. The mean age of the control group was (2.35±1.61) years,and there were 435 boys (57.39%). Overall vaccine effectiveness (VE) against all type influenza for two seasons combined was 58% (95%CI: 31%-74%). An analysis by age groups showed 68% (95%CI:41%-82%) of the VE estimate among children aged 36-72 months while it was 28%(95%CI:-80%-71%)of the VE estimate among children aged 6-35 months. The VE estimate value was 54% (95%CI:16%-75%) against all type influenza and 65% (95%CI:24%-83%) against influenza A (H(3)N(2)) during the 2016-2017 season. During the 2017-2018 season, the VE estimate value was 69% (95%CI:18%-88%) against all type influenza. Conclusion: Influenza vaccine is effective in preventing influenza virus infection during the flu season,especially the effect among children aged 36-72 months is higher compared to that among children aged 6-35 months.