Impact on Pediatric Respiratory Virus Epidemiology Following Nirsevimab Introduction

Background: – Nirsevimab prevents respiratory syncytial virus (RSV)-associated hospitalizations in infants. However, its impact on the broader epidemiology of acute respiratory infections (ARI) in hospitalized children remains unclear. Methods: – We conducted a retrospective study of ARI hospitalizations among children aged ≤5 years at a tertiary pediatric hospital in Parma, Emilia Romagna region, Italy, across 3 respiratory seasons (from September 1, 2022 to April 30, 2025), spanning the period before and after nirsevimab introduction (October 2024). All children hospitalized with ARI during the seasonal period (September–April) who tested positive for at least 1 respiratory virus by multiplex polymerase chain reaction were included. We analyzed temporal trends in ARI hospitalization rates, viral distribution and RSV-specific infection patterns. Hospitalization rates were contextualized using the resident population aged ≤5 years in Emilia Romagna. Results: – A total of 241 ARI hospitalizations were included, with 347 viral detections, of which 44.7% were monoinfections and 55.3% were coinfections. The incidence of ARI hospitalizations decreased in the 2024–2025 season compared with the previous 2 seasons (from 14.3 to 2.2 per 100 person-months). The overall distribution of respiratory viruses remained stable over time (P-trend = 0.4736), with RSV and rhinovirus accounting for approximately 35% and 30% of detections, respectively. The proportion of RSV monoinfections remained unchanged, whereas RSV coinfections declined significantly (P-trend = 0.0161). Conclusions: – Following nirsevimab introduction, pediatric ARI hospitalizations declined without evidence of viral replacement. The selective reduction in RSV coinfections suggests that RSV prevention may reduce infection complexity in addition to RSV-associated disease. Continued surveillance is needed to assess the long-term epidemiologic impact.