Impact of regional heterogeneity of RSV infection prophylaxis on bronchiolitis in Italy

Abstract Respiratory syncytial virus (RSV) remains a leading cause of bronchiolitis and hospitalization in infants across Italy, particularly during predictable seasonal outbreaks. With the 2023–2025 RSV seasons marking the first large-scale rollout of the monoclonal antibody nirsevimab, this study aimed to evaluate the implementation strategies, regional variability, and early clinical impact of RSV prophylaxis at the national level. We conducted a nationwide, multicenter, observational study across 19 Italian regions. Data were collected through the Italian Society of Pediatrics and the Italian Society of Neonatology, focusing on regional differences in rollout timing, organizational models, logistical challenges, and RSV-related hospitalizations and pediatric intensive care unit (PICU) admissions. The results highlighted significant regional heterogeneity. Northern and central regions such as Veneto, Lombardy, and Tuscany initiated prophylaxis earlier and experienced fewer logistical barriers, resulting in marked reductions in RSV hospitalizations - up to 83.7% in Friuli-Venezia Giulia. Conversely, southern and smaller central regions, including Molise, Marche, and Umbria, faced delayed starts, supply shortages, and bureaucratic challenges, leading to more modest decreases. PICU admissions mirrored these trends. Overall, the national introduction of nirsevimab correlated with a significant reduction in RSV-related morbidity, especially in regions with early, well-organized rollouts. Comparative international studies from France, Spain, and Luxembourg reinforce the Italian findings: timely and universal prophylaxis leads to substantial public health benefits. The study concludes that early campaign activation, consistent drug availability, and efficient organization are critical for maximizing the protective effect of RSV immunization.