Real-World Effectiveness of Universal Nirsevimab on Bronchiolitis Incidence: A Regional Hub-Spoke Surveillance...

Bronchiolitis represents a major cause of infant hospitalization, predominantly due to respiratory syncytial virus (RSV). Nirsevimab, a long-acting monoclonal antibody targeting the prefusion RSV F protein, has recently been introduced to provide season-long protection against RSV-related lower respiratory tract infection.

We conducted a multicenter, retrospective, population-based study across the Liguria Region (Northwest Italy) to assess the impact and effectiveness of the 2024-2025 nirsevimab immunization campaign. All infants under 2 years of age hospitalized with acute bronchiolitis between October 2023 and March 2025 were identified through the regional pediatric Hub-and-Spoke network. Data were compared between the pre-nirsevimab (2023-2024) and post-nirsevimab (2024-2025) seasons. Nirsevimab effectiveness (VE) against RSV-positive hospitalization was estimated using a test-negative design.

A total of 858 infants were hospitalized with bronchiolitis across the two seasons. Following the introduction of nirsevimab, bronchiolitis hospitalizations decreased from 41.7 to 17.3 per 1000 admissions (Incidence Rate Ratio 0.4; 95% CI: 0.4-0.5; p < 0.001), corresponding to a 60% reduction. Estimated nirsevimab effectiveness was 65.0% (95% CI: 39.2%-79.8%) against RSV-positive hospitalization. No high-risk infants-those previously eligible for palivizumab-developed severe RSV infection requiring PICU admission. Breakthrough infections occurred in a minority of immunized infants but showed a similar clinical course to non-immunized cases.

The introduction of universal nirsevimab immunization in Liguria was associated with a marked reduction in RSV-related hospitalizations, confirming its strong real-world epidemiological impact. These findings support nirsevimab as an effective and feasible public health intervention to substantially reduce the burden of RSV bronchiolitis in infants.