Impact of COVID-19 Pandemic on Bronchiolitis Epidemiology in Greece.

Background and Objectives: Acute bronchiolitis is a leading cause of hospitalization and morbidity in infants and young children. The COVID-19 pandemic has impacted its epidemiology globally. This study aims to assess changes in bronchiolitis epidemiology in Greece during the period of COVID-19 restrictive measures and after their removal, compared to the pre-pandemic period. Materials and Methods: A retrospective non-interventional study was conducted at a tertiary pediatric hospital, including all children aged 0-24 months hospitalized for bronchiolitis between 1 November 2017 and 30 September 2024. A total of 1505 cases were included. Data were analyzed across seven seasonal periods. Results: Bronchiolitis hospitalizations decreased by 98% in 2020-2021, returned to pre-pandemic levels in 2021-2022, and increased by 58.7% in January 2022-2023. The seasonal distribution shifted earlier in 2021-2022, with a return to pre-pandemic patterns in 2022-2023 and 2023-2024. No shift was observed in the peak age group (1-3 months), although neonatal hospitalizations decreased significantly in 2020-2021 (p = 0.009). RSV-positive cases dropped notably during 2020-2021 (41.2%). There was no change in disease severity across periods, assessed by indirect indicators and the Modified Tal Score. The proportion of bronchiolitis cases among total pediatric admissions increased to 5.6% in 2023-2024, compared to 4.9% in 2022-2023 and 3.9% pre-pandemic (2017-2020). Conclusions: Bronchiolitis epidemiology was significantly impacted by the COVID-19 pandemic in Greece, though disease severity remained unchanged. Ongoing monitoring of bronchiolitis and RSV circulation is crucial particularly with the introduction of maternal RSV vaccination and new monoclonal antibodies to inform prevention strategies and reduce disease burden.