Impact of Nirsevimab in Its Second Season on Respiratory Syncytial Virus and Non-RSV Admissions in Children Under 5

Background: Nirsevimab was incorporated into the universal infant immunization schedule in October 2023. While its effectiveness during the first season has been described, real-world data on the second season and its impact on other respiratory virus hospitalizations are still limited, though more studies are likely to forthcoming. We aimed to evaluate the effect of nirsevimab implementation on hospital and PICU admissions for acute lower respiratory tract infections, including respiratory syncytial virus (RSV) and non-RSV etiologies, in children under 5 years of age with a specific focus on the second postintroduction season. Methods: This retrospective study was conducted at a tertiary center in Tarragona, Spain. We included children under 5 hospitalized with acute lower respiratory tract infections (ALRTI) during the RSV seasons between 2018 and 2025 (excluding the 2020–2021 season due to the coronavirus disease pandemic). Demographic data, length of hospital stay, pediatric intensive care unit [PICU] stay and need for ventilatory support, were analyzed. Results: Among 1384 ALRTI hospitalizations, 661were RSV-confirmed and 375 required PICU admission. After nirsevimab rollout, hospital admissions due to RSV decreased from 5.04 to 2.5 per 1000 ( P < 0.001), hospitalizations due to non-RSV decreased from 5.1 to 3.54 per 1000 children <5 years ( P = 0.004) and PICU admissions due to ALRTI (RSV and non-RSV) declined from 2.9 to 1.3 per 1000 children ( P < 0.001). No differences were observed between the first and second postimplementation seasons in hospitalization, need for respiratory support, PICU or length of stay. Conclusions: Universal nirsevimab immunization was associated with sustained reductions in RSV-related hospitalizations and critical illness, but also in hospitalizations due to other respiratory viruses.