Respiratory Syncytial Virus Bronchiolitis Before and After COVID-19 Pandemic: Has the Immunity Debt Been Paid Off?

Background: Our aim was to hypothesize that the COVID-19 pandemic influenced the characteristics of viral bronchiolitis by comparing the last 3 epidemics with 3 pre-COVID-19 epidemics in infants hospitalized with bronchiolitis. Methods: We prospectively enrolled 637 consecutive infants (median age 3.0 +/- 2.1 months, 58.5% males), hospitalized for bronchiolitis during 6 consecutive annual epidemic seasons from 2017 to 2023. All parents of the children were given a structured anamnestic questionnaire. A nasopharyngeal aspirate was tested for 15 respiratory viruses. As measures of severity, we evaluated the O2 supplementation and the admission at the pediatric intensive care unit. Results: A total of 166 were hospitalized with bronchiolitis in 2017-2018, 97 in 2018-2019, 69 in 2019-2020, 0 in 2020-2021, 129 in 2021-2022 and 176 in 2022-2023. Taking together the 332 bronchiolitis cases hospitalized during the 3 prepandemic seasons, they peaked between December and January; after the flat curve in 2020-2021, the cases of bronchiolitis peaked in November 2021 and in December 2022. While the 2021-2022 season registered a less severe clinical presentation, O2 supplementation and pediatric intensive care unit admissions increased in 2022-2023 with respect to the prepandemic seasons (P < 0.001). Conclusions: This study represents an important scientific demonstration of the impact of primary prevention measures on the epidemiology of viral infections; their fluctuations were related to the intensity of restrictive measures and to the changing trend of respiratory viruses. It is essential to predict the real temporal trend of bronchiolitis not to leave high-risk children uncovered and to guide hospitals to maintain a high level of readiness.