Effectiveness of inactivated influenza and COVID-19 vaccines in hospitalized children in 2022/23 season in Japan - The first season of co-circulation of influenza and COVID-19.

We have analyzed the inactivated vaccine effectiveness (VE)for preventing influenza hospitalization by test-negative design in the 2022/23 season. This is the first season of co-circulation of influenza and COVID-19, and a unique period because all inpatients received COVID-19 screening. Among 536 children hospitalized with fever, none were positive for both influenza and SARS-CoV-2. The adjusted VE for preventing influenza A for all children, the 6-12-year-old group, and those with underlying diseases was 34 % (95 ％CI, -16 %-61 %, n = 474), 76 % (95 ％ CI, 21 %-92 %, n = 81), and 92 % (95 ％ CI, 30 %-99 %, n = 86), respectively. Only 1 out of 35 hospitalized cases with COVID-19, and 42 out of 429 controls, had been immunized with COVID-19 vaccine. This is the first report showing influenza VE by age group in children in this limited season. We still recommend the inactivated influenza vaccine for children based on the significant VE in subgroup analysis.