Impact of Nirsevimab on RSV and Non-RSV Severe Respiratory Infections in Hospitalized Infants

Abstract

Background: Nirsevimab, a monoclonal antibody providing passive immunity against RSV infections in infants, was introduced in Spain in October 2023 for children under 6 months and those born during the epidemic season. This study aimed to compare the clinical and virological characteristics of respiratory infections in hospitalized infants before and after nirsevimab introduction. Methods: We carried out a prospective study across two hospitals in Madrid during the 2022–2023 and 2023–2024 epidemic seasons. The study included infants under 12 months of age that were hospitalized with lower respiratory tract infections (LRTIs). Clinical, epidemiological, and virological data were analyzed and compared between the periods before and after the introduction of nirsevimab, as well as according to whether the infants had received this preventive treatment. Results: A total of 669 infants were included: 480 from October 2022 to March 2023 (S1) and 189 from October 2023 to March 2024 (S2). Respiratory infection–related admissions decreased by 62.5% in S2, with a 74.5% reduction in ICU admissions. RSV-related admissions decreased by 78%, HMPV by 36.6%, and adenovirus by 69.5%. Infants in S2 were older (p = 0.001) and had shorter hospital stays (p < 0.001) than in S1. Of 63 (33%) infants in S2 who received nirsevimab, 11 (17%) were diagnosed with RSV. High-flow oxygen use was less frequent among RSV patients treated with nirsevimab (p = 0.002). Conclusions: Nirsevimab introduction was significantly associated with reduced hospitalizations and severity of RSV and other respiratory infections. Its use was associated with fewer admissions and reduced need for intensive care, especially in RSV-infected infants but also in HMPV and adenovirus-infected infants.