Search

• After COVID-19, respiratory syncytial virus (RSV) caused intense peaks in cases. • The fall 2021 epidemic was mostly caused by RSV-A circulating also before COVID-19. • RSV-B had a greater occurrence in 2022–2023, as documented in other European nations. • Unlike RSV-A, RSV-B divergent strains had a mutational pattern not found before COVID. • RSV-B novelty and the immunity debt may have increased the burden of 22–23 epidemic. Following the pandemic restrictions, the epidemiology of respiratory syncytial virus (RSV) has changed, leading to intense hospitalization peaks. This study, conducted at multiple sites in Italy, aimed to describe the temporal dynamics of two post-COVID-19 RSV epidemics. Additionally, the circulating RSV-A and -B lineages were characterized and compared to those found in 2018 and 2019. Respiratory specimens and data were collected from RSV-positive patients, both inpatients, and outpatients, of all ages at three sites in north-central Italy. To analyze these samples, roughly one-sixth were sequenced in the attachment glycoprotein G gene and subjected to phylogenetic and mutational analyses, including pre-pandemic sequences from north-central Italy. The first post-pandemic surge of RSV cases was quite intense, occurring from October 2021 to early January 2022. The subsequent RSV epidemic (from November 2022 to early March 2023) also had a high impact, characterized by a rise in elderly patient cases. Post-pandemic cases of RSV-A were caused by various strains present in Italy prior to COVID-19. In contrast, a distinct RSV-B lineage, which was concurrently spreading in other countries, was identified as the main cause of the surge in 2022–2023 but remained undetected in Italy before the pandemic. This study describes the temporal dynamics of post-pandemic RSV subgroups and uncovers a lineage of RSV-B with high genetic divergence that may have increased the impact of decreased population immunity. [ABSTRACT FROM AUTHOR]