Inconsistent Increase in Age at Respiratory Syncytial Virus Hospitalization of Children Aged <2 Years During the Severe Acute Respiratory Syndrome Coronavirus 2 Pandemic: A Retrospective Multicenter Study in 4 European Countries

Background. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic disrupted respiratory syncytial virus (RSV) seasonality. To optimize the use and evaluation of RSV infant immunization strategies, monitoring changes in RSV epidemiology is essential. Methods. Hospitalizations for acute respiratory infections (ARIs) and RSV-coded ARI in children <2 years were extracted in 4 European hospitals, according to predefined case definitions (International Classification of Diseases, Tenth Revision codes). Prepandemic RSV seasons (2017–2018 to 2019–2020) were compared to 2021–2022 and 2022–2023. Results. In 2021–2022 and 2022–2023, the peak number of RSV hospitalizations was higher than prepandemic peaks after short periods of RSV circulation, and lower than prepandemic peaks after long periods of RSV circulation. A greater proportion of RSV hospitalizations occurred in children 1 to <2 years in 2021–2022 in the Netherlands (18% vs 9%, P = .04). No increase in age was observed elsewhere. High-risk children represented a greater proportion of RSV hospitalizations during the pandemic. The proportion of pediatric intensive care unit admissions did not increase. Conclusions. A decrease in population immunity has been linked to older age at RSV hospitalization. We did not observe an increase in age in 3 of the 4 participating countries. Broad age categories may have prevented detecting an age shift. Monitoring RSV epidemiology is essential as Europe implements RSV immunization.