Coverage, Timing, Safety, and Determinants of Neonatal Intensive Care Unit Admission in Newborns Receiving Nirsevimab Prophylaxis for Respiratory Syncytial Virus

Background: Nirsevimab, a long-acting monoclonal antibody targeting respiratory syncytial virus (RSV), has recently been introduced as universal prophylaxis in newborns and infants. Although clinical trials have shown high efficacy and a favorable safety profile, real-world evidence remains limited. Objectives: Our Italian multicenter observational study included newborns eligible for nirsevimab prophylaxis during the 2025–2026 season and aimed to assess nirsevimab coverage, timing of administration, and short-term safety in routine clinical practice. The association between perinatal and environmental factors and neonatal intensive care unit (NICU) admission was also investigated, while RSV occurrence was evaluated as a descriptive and exploratory outcome. Methods: Eligible newborns were from four Italian hospitals during the 2025–2026 RSV season. We assessed nirsevimab coverage, timing of administration, RSV incidence, adverse events, and selected neonatal, perinatal, and maternal risk factors. Results: Among 1554 eligible newborns, 1462 received nirsevimab, corresponding to a coverage rate of 94.0%. The median age at administration was 3.0 days (IQR 2-6 days), with significantly delayed prophylaxis in preterm compared with term infants. During the observation period, two laboratory-confirmed RSV-associated bronchiolitis cases occurred, corresponding to a cumulative incidence of 0.13%. One case occurred in the immunized group and one in the non-immunized group. No adverse events following immunization were recorded. Conclusions: Nirsevimab prophylaxis achieved high coverage and showed a reassuring short-term safety profile in this real-world multicenter cohort. RSV infections were uncommon, and their very limited number prevents definitive conclusions regarding comparative effectiveness between immunized and non-immunized newborns. Prematurity was associated with delayed administration and greater need for intensive care, underscoring the importance of timely prophylaxis in vulnerable newborns.