Ongoing Shift Toward an Earlier RSV Season in Children: The Case of Saitama After the COVID-19 Pandemic.

Epidemics of respiratory syncytial virus (RSV) infection previously followed a predictable and seasonal pattern. However, the emergence of COVID-19 and the accompanying public health measures disrupted RSV seasonality worldwide. To investigate the long-term epidemiological impact, we analyzed weekly RSV-related hospitalizations in children under 15 years of age from 2018 to 2024 in Saitama Prefecture, Japan. During the COVID-19 pandemic, RSV admissions notably declined in 2020, but the number of hospitalizations returned to pre-pandemic levels. The peaks of admission rates occurred in March, in both 2023 and 2024, which were earlier than the August peaks seen in 2018 and 2019. We also found that the proportion of hospitalized patients aged ≥2 years increased notably to 30% in 2021 and 2022, and declined to 21.2% in 2024. Although an upward trend in intubation rates was observed, no statistically significant differences were found. These findings suggest that the COVID-19 pandemic led to earlier RSV outbreaks and temporary shifts in age distribution. It remains unclear whether RSV seasonality and age patterns will return to their pre-pandemic status. Ongoing surveillance is essential to better predict future outbreaks and inform optimal timing for preventive interventions, such as vaccination or monoclonal antibody administration.