Recent trends in hospitalizations for respiratory syncytial virus after the COVID-19 pandemic and before routine immunization: Seasonality and severity updates from the 2023/2024 season in Tuscany, Italy

Objectives: Respiratory syncytial virus (RSV) epidemiology was disrupted by the COVID-19 pandemic. This study investigates the 2023/2024 RSV season in Tuscany, Italy, to assess trends in seasonality and incidence of RSV-associated hospitalizations compared with pre-pandemic and prior post-pandemic seasons. Methods: We analyzed RSV-associated hospitalizations in Tuscany during the 2023/2024 season, just before the implementation of routine immunization with nirsevimab, with a dynamic cohort consisting of all resident children aged ≤2 years, using regional registries. Seasonality was assessed using the 60% mean detection threshold method, and incidence rate ratios and risk ratios were used to compare hospitalization incidence and risk of severe hospitalization (intensive care unit, continuous positive airway pressure, or mechanical ventilation) with pre-pandemic seasons, 2017/2018 and 2018/2019, respectively. Results: Among 64,963 children aged <2 years, 724 were hospitalized for RSV. The epidemic began in week 42 of 2023, peaked in week 52, and ended in week 12 of 2024 (18 weeks total). Incidence remained significantly higher than pre-pandemic levels (incidence rate ratio 3.0, 95% confidence interval 2.7-3.4), whereas severity risk was comparable to pre-pandemic seasons. Conclusions: Seasonality in 2023/2024 aligned more closely with pre-pandemic patterns, but incidence remained elevated, likely due to immunity debt at the population level. Monitoring RSV epidemiology is essential as new preventive measures, such as nirsevimab and vaccines, are introduced.