Temporal dynamics and variants of respiratory syncytial virus causing bronchiolitis after COVID-19

What is it about?

In the autumn 2021, respiratory syncytial virus (RSV) returned to circulate after easing of pandemic restrictions and caused a number of hospitalizations for bronchiolitis far higher than in the past. In Rome, this surge of cases was mainly caused by RSV variants of subtype A that circulated also before COVID-19 pandemic. The infants infected with these RSV-A variants experienced bronchiolitis severity similar to previous epidemic seasons. The intense RSV peak in 2021-2022, can be attributable to the lack of infections in 2020-early2021 that decreased RSV immunity at the population level, creating a sort of “immune debt”. In autumn/winter 2022-2023, a high number of hospitalizations for bronchiolitis was also recorded in Rome, but they were mainly caused by infections with RSV-B variants carrying amino acid substitutions in the G glycoprotein not detected before 2020. RSV-B infected infants hospitalized for bronchiolitis were more frequently admitted to intensive care units and needed O2 in 2022-2023, with respect to the previous RSV seasons. The RSV-B genetic divergence observed in post-pandemic strains may have increased the RSV-B specific immune debt, being a possible contributor to bronchiolitis severity in 2022-2023.

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Why is it important?

Studies on SARS-CoV-2 variants have provided an impetus to investigate as-yet-unknown aspects of respiratory syncytial virus (RSV), the main cause of infants’ hospitalization. Our study adds new elements to the understanding of the pathogenetic mechanisms associated with RSV variants circulating in the post-pandemic period. There is a need for increased epidemiological surveillance of RSV at the national level and genomic sequencing projects at the local level, which can monitor infectivity and pathogenicity of RSV variants and predict the intensity of seasonal peaks of bronchiolitis cases in order to rationalize healthcare resources.