Respiratory Syncytial Virus Infection Notification Trends and Interpretation of the Reported Case Data, 2018-2021, Japan.

In Japan, as elsewhere, the coronavirus disease 2019 (COVID-19) pandemic has affected notification trends of respiratory syncytial virus (RSV) infection. Here, we describe the epidemiological trends of cases of RSV infection among children reported during 2018-2021 in Japan based on the national surveillance system. Compared with 2018 and 2019, 2020 saw an unprecedented decrease in notifications of RSV infection per sentinel site. However, 2021 experienced an unseasonably early and high peak at week 28 (peak week in 2018 and 2019 was week 37), with a large resurgence in notifications nationwide and across regions. Regarding age, compared with 2018 and 2019, the number and proportion of case-patients aged 2, 3, and ≥4-years increased substantially in 2021, but the number of case-patients aged <1 year decreased slightly. Furthermore, in 2021, the ratio of notifications per site from outpatient clinics to notifications per site from hospitals increased, suggesting a proportionate increase in diagnoses of clinically milder cases. Notably, RSV-attributed deaths from vital statistics also dropped substantially in 2020 and rebounded in 2021 but were fewer than in 2018 or 2019. While the incidence of RSV infections likely declined in 2020 (possibly owing to COVID-19 countermeasures) and increased in 2021, notifications in 2021 appeared to be associated with milder presentations. Given the unpredictable epidemiology of RSV, continuous monitoring and pluralistic assessments are imperative.