Background: For preventive strategies and new treatment options against RSV infection, which annually affects around three million children worldwide, knowledge about seasonality and disease severity is a prerequisite.

Objectives: The aim of this study was to provide clinical and epidemiological information on RSV infections before and during covid-19 pandemic (2015-22). Results should help to identify target populations and the right time for prophylaxis.

Methods: We retrospectively evaluated a total of 661 cases of laboratory confirmed RSV infections in hospitalized children by comparing five pre-covid-19 seasons with the covid-19 season 2021/22.

Results: In the pre-covid years RSV seasons started in December with the peak in January/February. In contrast, during the pandemic we detected no case in 2020/21 and then the first RSV cases in 8/2021; peak of season was 10/2021. When focusing on the most affected age group below 1 year, we found significantly higher incidences in 2021/22 and less need for intensive care compared to pre-covid seasons. The cumulatively analyzed pre-covid cohort, consisting of 398 cases appearing from 2015-2019, shows a significantly longer duration of hospital stay and x-rays were performed more frequently compared to the covid-cohort from 2021/22, consisting of 263 cases.

Conclusion: Our results indicate that the complete lack of cases from 05/2020 to 06/2021 stimulated a catch-up effect in season 2021/22 with significantly higher RSV incidence and associated with lower disease severity, especially in infants. The early onset of the RSV season during the pandemic should be considered for future vaccination strategies.