Real-World Effectiveness of Nirsevimab Against RSV-LRTI and Bronchiolitis Hospitalization: Results from the 2024–2025 Italian Universal Immunization Campaign in Newborns

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Respiratory syncytial virus (RSV) remains a leading cause of lower respiratory tract infection (LRTI) and hospitalization in early infancy. Following the introduction of universal nirsevimab prophylaxis in Italy during the 2024–2025 RSV season, we evaluated its real-world effectiveness against RSV-associated hospitalization and severe clinical outcomes within a test-negative design framework. We conducted a multicenter retrospective test-negative study across 88 Italian neonatal centers, including infants hospitalized for LRTI from 01/09/2024 to 30/04/2025. Clinical data were collected from hospital records using a standardized electronic questionnaire. RSV-positive hospitalized infants were considered cases, and RSV-negative hospitalized infants served as controls. Secondary outcomes included respiratory support and Neonatal Intensive Care Unit (NICU) admission. Effectiveness was estimated using multivariable logistic regression and targeted maximum likelihood estimation. Overall, 737 infants were included, of whom 306 (41.52%) had previously received nirsevimab. RSV infection was identified in 398 infants (54.00%), including 104/306 (33.99%) nirsevimab-immunized and 294/431 (68.21%) non-immunized infants (p < 0.001). Immunized infants also required respiratory support less frequently (55.23% vs. 70.07%; p < 0.001), with lower use of high-flow oxygen therapy (40.85% vs. 48.96%; p = 0.036) and Continuous Positive Airway Pressure (CPAP) (18.95% vs. 33.87%; p < 0.001). Within the test-negative design framework, adjusted effectiveness against RSV-associated hospitalization was estimated at 78.3%. Among hospitalized infants, previous nirsevimab immunization was additionally associated with adjusted relative reductions in the odds of respiratory support and NICU admission of 80.2% and 64.3%, respectively. In conclusion, among infants hospitalized for LRTI in routine Italian clinical practice, previous nirsevimab immunization was associated with lower odds of RSV positivity and a reduced need for respiratory support. Although these findings provide further evidence of the real-world effectiveness of nirsevimab against severe RSV disease, further studies are needed to provide direct estimates of hospitalization incidence in immunized and non-immunized source populations.