Post-pandemic immunity debt for paediatric infectious diseases in Japan between 2014 and 2024

Non-pharmaceutical interventions (NPIs) imposed during the COVID-19 pandemic were known to create an immunity debt.This study aimed to quantify the immunity debt for respiratory syncytial virus (RSV), adenovirus, group A streptococcus (GAS), and influenza in Japan between 2014 and 2024.We conducted a retrospective observational study using national surveillance data and electronic health records in 23 clinics.We conducted an interrupted time-series analysis using a quasi-Poisson regression model to estimate the counterfactual incidence that would have occurred in the absence of NPIs.The number of RSV cases declined by 84% in the 2020-2021 season and increased by 39% in the 2021-2022 season, primarily due to an increase in cases among 2-year-old children (94%).Adenovirus, GAS, and influenza were suppressed during the 2020-2022 season by 58-99%.Adenovirus cases increased by 195% in 2023, with a 458% increase among children aged 5-9 years.GAS increased by 36% in 2024, with a 96% increase among 10-14-year-olds.Influenza increased by 158% in 2023, with a 299% increase among 10-14-year-olds.Surveillance data and data from 23 clinics showed similar trends.The study suggests that the intensity and timing of the immunity debt among the paediatric population differed by pathogen.