Seasonality and severity of respiratory syncytial virus during the COVID-19 pandemic: a dynamic cohort study

Objectives: to investigate seasonality, epidemiological characteristics, and clinical severity variations of RSV-associated hospitalizations following the easing of COVID-19 restrictions in Tuscany, Italy, up to the 2022-2023 season. Methods: from 2017 to 2023, a dynamic cohort consisting of all resident children aged ≤2 years was followed-up in regional registries. Person-time incidence rate(IR) of RSV-associated hospitalizations per 1,000 person-years and risk of severe hospitalization (ICU, C-PAP, or mechanical ventilation) per 100 RSV hospitalizations were calculated. RSV seasonality was investigated with retrospective methods. Results: in total, 193,244 children were followed-up. After the easing of restrictions, RSV epidemics showed earlier seasonality and shorter duration compared to pre-pandemic (2017 to 2019), with this deviation decreased in 2022-2023. In 2021-2022 and 2022-2023, the IR of RSV-associated hospitalizations significantly increased compared to pre-pandemic (2022-2023 risk ratio [RR]: 3.6, 95%CI 3.3-4.0), with larger increases among older age groups. Among hospitalized children, only those aged ≥12 months showed an increased risk of severe hospitalization, particularly during the 2021-2022 (RR 4.7, 95%CI 1.5-24.3). Conclusions: findings suggest a gradual return of RSV epidemics to the pre-pandemic pattern, although relevant increases in disease incidence persist. Reduced regular RSV exposure among older children may lead to declining immunity and increased severe outcome risks.