Impact of Seasonal Nirsevimab Administration in Infants Born During the RSV Circulation Period on RSV-Related Hospitalizations: A Population-Based Study from Emilia-Romagna, Northern Italy

Background: Respiratory syncytial virus (RSV) is a leading cause of hospitalization in early infancy, with the greatest burden occurring in the first months of life. Following the COVID-19 pandemic, many countries experienced intensified RSV circulation. Nirsevimab, a long-acting monoclonal antibody providing season-long protection after a single dose, was introduced in Italy for the 2024–2025 RSV season and recommended for infants born during the period of RSV circulation. We evaluated the population-level impact of this seasonal nirsevimab strategy on RSV-related hospitalizations among young infants. Methods: We conducted a population-based observational study using regional hospital discharge records from Emilia-Romagna, Northern Italy, spanning January 2017 to April 2025. Analyses were restricted to RSV seasons (October–March) and infants aged ≤180 days. RSV-related hospitalizations were identified using ICD-9-CM codes. Hospitalization rates were calculated per 100,000 person-days. Incidence rate ratios (IRRs) were estimated using negative binomial regression models adjusted for season, age group, and sex, with clustering at the hospital level. The post-nirsevimab season (2024–2025) was compared with the immediate pre-nirsevimab season (2023–2024) and a pre-COVID reference season (2018–2019). Results: A total of 551 RSV hospitalizations occurred in the pre-COVID season, 753 in the pre-nirsevimab season, and 252 in the post-nirsevimab season. The post-nirsevimab season was associated with a substantial reduction in RSV-related hospitalization rates compared with both the pre-COVID season (IRR 0.52; 95% CI 0.41–0.66) and the pre-nirsevimab season (IRR 0.36; 95% CI 0.29–0.44). Reductions were observed consistently across epidemic months and were most pronounced during the first three to four months of life. Conclusions: Seasonal administration of nirsevimab to infants born during the RSV circulation period was associated with a marked and sustained reduction in RSV-related hospitalizations in early infancy. These findings support the effectiveness of targeted, seasonally timed infant immunoprophylaxis as a population-level RSV prevention strategy.