Heterogeneous reduction in pediatrics RSV hospitalizations following regional nirsevimab immunization strategies: a multicenter study

Objectives To describe pediatrics RSV-hospitalization trends over six seasons and changes following regional nirsevimab strategies in Italy. Methods A retrospective study across 19 Italian pediatrics units were conducted analyzing RSV-related hospitalizations in patients aged <18 years from January 2019 to March 2025. Centers were grouped by regional nirsevimab eligibility criteria (Groups A-G). Group-level ratios of 2024-2025 to preimmunization hospitalizations were estimated with a negative-binomial model. Influenza was analyzed as a negative control Results 10,915 RSV hospitalizations were recorded. In 2024-2025 RSV season, hospitalizations decreased 43.6% overall. The largest reductions were in groups with broader eligibility and earlier initiation (Group B: ratio 0.28, 95% CI 0.22-0.36; Group A: 0.40, 95% CI 0.22-0.74) and Group F (0.24; single center, interval indicative). Groups C, D, and E showed more modest reductions (ratios 0.58-0.70), with Group C spanning unity (0.58, 95% CI 0.27-1.25). Group G showed no clear change once baseline instability was accounted for (1.84, 95% CI 1.15-2.94). Conclusions These population-level study describe a temporal association between the 2024 and 2025 nirsevimab rollout and reduced RSV hospitalizations, particularly with broader eligibility and timely rollout. The study design cannot establish causation or compare the effectiveness of regional strategies.