Effectiveness of inactivated influenza vaccine in children during the 2023/24 season: The first season after relaxation of intensive COVID-19 measures

Background: The annual administration of the influenza vaccine is the most effective method for preventing influenza. We have evaluated the effectiveness of the inactivated influenza vaccine in children aged 6 months to 15 years across the seasons from 2013/2014 to 2022/2023. This study aims to investigate the effectiveness of the inactivated influenza vaccine in the 2023/2024 season, the first year following the easing of strict COVID-19 measures, and possibly the last season when only the inactivated vaccine is available on the market. Methods: Adjusted vaccine effectiveness for the 2023/2024 season was assessed using a test-negative case-control design, with results based on polymerase chain reaction and rapid influenza diagnostic tests. Vaccine effectiveness was calculated by influenza type and patient hospitalization/outpatient status. Results: A total of 1832 children were recruited. The inactivated influenza vaccine was effective in preventing both symptomatic influenza A and B in both inpatient and outpatient settings. Overall vaccine effectiveness for influenza A was 51% (95% confidence interval [CI], 23%–69%, n = 930) in inpatient settings and 54% (95%CI, 27%–71%, n = 559) in outpatient settings. For influenza B, effectiveness was 60% (95%CI, 22%–79%, n = 859) in inpatient settings and 56% (95%CI, 26%–74%, n = 558) in outpatient settings. Analysis suggested that administering two doses enhanced effectiveness specifically against influenza B. Conclusions: This is the first study to demonstrate influenza vaccine effectiveness in children after the relaxation of strict COVID-19 measures in Japan (2023/2024). We recommend the current inactivated vaccine for preventing both influenza A and B in children, with consideration for the potential use of two doses to enhance effectiveness against influenza B.