# Epidemiology of moderate-to-severe respiratory syncytial virus infections in children in subtropical Okinawa, Japan: a 4-year retrospective study

## TL;DR

This study examines the patterns and risk factors of severe RSV infections in young children in subtropical Okinawa, Japan, over four years, including changes during the early pandemic.

## Contribution

The study provides the first comprehensive analysis of RSV epidemiology in subtropical Japan, highlighting local risk factors and seasonal trends.

## Key findings

- Severe RSV cases were most common in children under 24 months, with the highest burden in infants aged 0–2 months.
- Younger age, presence of siblings, and multiple underlying diseases were independent risk factors for severe RSV infection.
- RSV seasonality shifted during the pandemic, with broader epidemic curves and reduced case numbers compared to pre-pandemic years.

## Abstract

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 5 years across Okinawa.

This retrospective, multicenter study analyzed pediatric cases of laboratoryconfirmed 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 diseases, household and childcare characteristics, and seasonal trends were assessed.

A total of 1541 hospitalized RSV cases were included, of which 117 (7.6%) were classified as severe. Overall, 89.0% were under 24 months of age, with the highest burden in the 0–2 month group. In univariate analysis, severe cases were significantly younger, more likely to have siblings, and less likely to attend nursery school compared with moderate cases. The overall prevalence of underlying diseases did not differ between groups; however, having two or more underlying diseases was significantly associated with severity. Multivariate logistic regression confirmed younger age, the presence of siblings, and underlying diseases (both any and multiple) as independent risk factors for severe infection. Seasonal peaks occurred consistently in summer during 2017–2019. In contrast, in 2020, coinciding with the onset of the COVID-19 pandemic, the epidemic curve became broader and peak timings varied across hospitals. Overall, the total number of cases decreased by 62% compared with the pre-pandemic average.

This study provides a comprehensive 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.