Objective:

To describe pediatric RSV hospitalization trends over six seasons and assess early population-level changes during the 2024–2025 season across Italian regions with different nirsevimab implementation strategies.

Approach:

• Study Design: A retrospective, observational, descriptive, multicenter study examined RSV-related hospitalizations among patients younger than 18 years across 19 pediatric units in 12 Italian regions from January 1, 2019, through March 31, 2025.
• Data Sources: Hospitalizations were identified using RSV-specific ICD-9-CM diagnostic codes. The analysis used fully anonymized, aggregated hospital administrative data without individual clinical information or patient identifiers.
• Nirsevimab Implementation: The participating centers were divided into seven groups according to regional birth month-based eligibility criteria and the timing of nirsevimab campaign initiation during the 2024–2025 season.
• Statistical Analysis: Hospitalization counts during 2024–2025 were compared with the mean counts from the three preceding preimmunization seasons using negative-binomial regression. Influenza hospitalizations were examined as a negative control, and additional sensitivity analyses were performed.

Key Findings:

• Among 10,915 RSV-related hospitalizations recorded during the study period, hospitalizations decreased by 43.6% overall during the 2024–2025 season compared with the three-season preimmunization mean.
• The largest modeled reductions among multicenter groups occurred in Groups B and A, which had broader eligibility and earlier implementation. Group F had the largest point reduction but included only one center and used narrower eligibility with a later start.
• Groups C, D, and E had more modest reductions, while Group G showed no clear reduction after instability in its preimmunization baseline was considered.
• Influenza hospitalizations increased by 93.7% across the same centers, arguing against general changes in admission thresholds or diagnostic coding as the primary explanation for the decline in RSV hospitalizations.

Interpretation:

The findings show a temporal association between the 2024–2025 nirsevimab rollout and lower pediatric RSV hospitalization counts. The pattern is compatible with broader eligibility and timely implementation contributing to larger reductions, but unmeasured uptake, baseline instability, and center-level differences prevent causal attribution or direct comparisons of regional effectiveness.

Limitations:

• The observational, uncontrolled before-and-after design cannot establish causation or compare the effectiveness of regional strategies.
• Aggregated data prevented age-specific analyses, confirmation of individual immunization status, and measurement of regional or center-level nirsevimab uptake.
• The analysis lacked precise population denominators and measured changes in hospitalization counts rather than true incidence rates.
• RSV identification relied on administrative coding, and the post-COVID preimmunization seasons may not represent a stable historical baseline.

Conclusion:

The 2024–2025 nirsevimab rollout was temporally associated with reduced pediatric RSV-related hospitalizations in Italy, although reductions varied across regional implementation groups. Continued surveillance, harmonized coverage monitoring, and age-specific analyses are needed to assess nirsevimab’s impact and guide timely, equitable RSV prevention strategies.