Epidemiology of Moderate-to-Severe Respiratory Syncytial Virus Infections in Children in Subtropical Okinawa, Japan: A Four-Year Retrospective Study

Background: Respiratory syncytial virus (RSV) is a leading cause of acute lower respiratory tract infections (ALRTIs) in infants and young children worldwide. While its epidemiology is well-characterized in temperate climates, data from subtropical regions such as Okinawa, Japan, remain limited. This study aimed to describe the clinical and demographic characteristics, risk factors, and seasonality of moderate-to-severe RSV infections in children under five years across Okinawa. Methods: This retrospective, multicenter study analyzed pediatric cases of laboratory-confirmed RSV infection requiring hospitalization between April 2017 and March 2021. Data were collected from four core hospitals across Okinawa Prefecture. Patients were categorized as having moderate or severe disease based on ICU admission status. Demographic variables, underlying medical conditions, household and childcare characteristics, and seasonal trends were assessed. Results: A total of 1,541 hospitalized RSV cases were included, of which 117 (7.6%) were classified as severe. Approximately 60% of cases were in infants under 12 months, with the highest burden in the 0–2 month age group. The severe group had a significantly lower median age and higher proportion of children with siblings. Nursery school attendance was more common in the moderate group, likely reflecting the younger age of severe cases. While the prevalence of individual comorbidities did not differ between groups, having multiple underlying conditions was significantly associated with disease severity. RSV activity peaked during summer (June–August) in 2017–2019 but shifted to winter in 2020–2021, coinciding with the COVID-19 pandemic and a 62% reduction in cases. Conclusions: This study offers the first region-wide assessment of moderate-to-severe pediatric RSV infections in a subtropical setting in Japan. Despite not including data on the use of palivizumab, nirsevimab, or maternal vaccination, the findings provide essential baseline data to guide the implementation of new preventive strategies tailored to local epidemiology.