Changes following the Onset of the COVID-19 Pandemic in the Burden of Hospitalization for Respiratory Syncytial Virus Acute Lower Respiratory Infection in Children under Two Years: A Retrospective Study from Croatia.

To understand the changes in RSV hospitalization burden in children younger than two years following the onset of the COVID-19 pandemic, we reviewed hospital records of children with acute lower respiratory infection (ALRI) between January 2018 and June 2022 in Split-Dalmatia County, Croatia. We compared RSV activity, age-specific annualized hospitalization rate, and disease severity between pre-COVID-19 and COVID-19 periods. A total of 942 ALRI hospital admissions were included. RSV activity remained low for the typical RSV epidemic during 2020-2021 winter. An out-of-season RSV resurgence was observed in late spring and summer of 2021. Before the COVID-19 pandemic, the annualized hospitalization rate for RSV-associated ALRI was 13.84/1000 (95% CI: 12.11-15.76) and highest among infants under six months. After the resurgence of RSV in the second half of 2021, the annualized hospitalization rate for RSV-associated ALRI in children younger than two years returned to the pre-pandemic levels with similar age distribution but a statistically higher proportion of severe cases. RSV immunization programs targeting protection of infants under six months of age are expected to remain impactful, although the optimal timing of administration would depend on RSV seasonality that has not yet been established in the study setting since the onset of the COVID-19 pandemic.