Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study

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Introduction Respiratory Syncytial Virus (RSV) is the leading cause of lower respiratory tract infections (LRTIs) in children. In 2024, the monoclonal antibody nirsevimab became available in Italy for all newborns during their first RSV season. Aim To evaluate whether there were changes in the circulation of respiratory viruses among hospitalized children under five years of age with viral LRTI. Secondarily, whether the introduction of nirsevimab influenced the hospitalization rate. Methods A retrospective study was conducted, comparing two epidemic seasons: pre-nirsevimab (2023–2024) and post-nirsevimab (2024–2025).All children under five years of age hospitalized for viral LRTIs were included and divided in bronchiolitis and “other-LRTIs”. All hospitalized patients underwent nasopharyngeal swab testing using multiplex-PCR for respiratory viruses’ detection. Results A total of 278 patients were enrolled. RSV circulation decreased in the post-nirsevimab season, both in the bronchiolitis and LRTI groups, although it remained the main cause of infections. In the post-nirsevimab season, detections of influenza virus and bocavirus increased in the bronchiolitis cohort (p-values 0.02 and 0.01), while human metapneumovirus and coronaviruses were more frequently identified in children with LRTIs (p-values 0.08 and 0.03). Regarding infants with bronchiolitis, we showed a 59% reduction in hospitalizations rate (both in paediatric ward and paediatric intensive care) ( p -value < 0.001). Children hospitalized for bronchiolitis were older than the previous season (p < 0.001). No differences were observed in the hospitalization rate among infants with viral LRTI. Conclusions Following nirsevimab introduction, circulation of non-RSV respiratory viruses increased, while hospitalizations and ICU admissions for bronchiolitis significantly decreased.

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