Impact on Pediatric Respiratory Virus Epidemiology Following Nirsevimab Introduction

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.

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.

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).

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.