Impact of Nirsevimab on bronchiolities in pediatric primary care in Lazio Region: an observational study

Background Bronchiolitis is the leading cause of lower respiratory tract infections in infants, with respiratory syncytial virus (RSV) as the primary pathogen. Nirsevimab, a long-acting monoclonal antibody, was introduced in Europe in late 2022 for RSV prophylaxis in all infants. In Italy, the 2024–2025 immunization campaign faced regional disparities in implementation. This study aimed to evaluate the impact of Nirsevimab in reducing bronchiolitis diagnoses and hospitalizations in infants during their first RSV season in the Lazio Region. Methods We conducted a retrospective cohort study using data from 29 primary care pediatricians in Lazio. Infants born between August 17, 2024, and March 31, 2025, were included. Bronchiolitis diagnoses were clinically defined, and immunization status was obtained from the regional vaccination registry. We excluded premature infants (< 33 weeks) and those diagnosed before the immunization campaign began on December 9, 2024. To adjust for confounding, we applied inverse probability weighting (IPW) based on a propensity score including demographic and clinical covariates. A negative binomial mixed-effects model was used to estimate incidence rate ratios (IRRs) for bronchiolitis and hospitalizations. Results Among 818 eligible infants, 613 (74.9%) were immunized. A total of 58 bronchiolitis cases were recorded (7.1%), with 6.5% in the immunized group and 8.8% in the non-immunized group. Crude analysis showed a 25.7% risk reduction, while IPW-adjusted analysis indicated a 50.4% reduction (95% CI: 44.4%–55.7%). Hospitalizations occurred in 2.0% of infants, with adjusted analysis showing a 49.1% reduction in hospitalization risk among immunized infants. The immunization campaign’s late start likely limited its full impact, as 40 early-season cases were excluded. Conclusions Nirsevimab immunization significantly reduced the risk of bronchiolitis and related hospitalizations in a real-world primary care setting. This result aligns with previous studies, though it may be underestimated due to the delayed campaign start and inclusion of all-cause bronchiolitis. These findings support early and widespread implementation of Nirsevimab to optimize protection and reduce RSV burden in infants.