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Nirsevimab Effectiveness Against Rsv Hospital Admission in Children Under 1 Year of Age: A Spanish Population-Based Case Control Study
Background: Respiratory Syncytial Virus (RSV) causes substantial morbidity in infants <1 year. In October 2023, Spain recommended the monoclonal antibody nirsevimab to all children born since 1 April 2023, at birth or as catch-up if born before October 2023. We estimated nirsevimab effectiveness in preventing RSV hospitalisations during the 2023-24 season.
Methods: We conducted a nationwide population-based matched case-control study. Cases were children hospitalised for lower respiratory tract infection who were RSV PCR-positive. For each case, four population-density controls born in the same province and date (±2 days) were selected. We defined at-birth immunisation as receiving nirsevimab during the first 2 weeks of life, and catch-up immunisation within 30 days from campaign onset. Causal intention-to-treat (ITT) and per-protocol (PP) effectiveness was estimated using inverse-probability-of-immunisation weighted conditional logistic regression.
Findings: We included 427 cases and 1707 controls in catch-up and 606 cases and 2421 controls in at-birth immunisation studies. Effectiveness in preventing RSV hospitalisations (95% confidence interval) for catch-up immunisation was 71% (65-77%) by ITT and 81% (76-85%) PP. Effectiveness for at-birth immunisation was 80% (76-84%) by ITT and 84% (80-87%) PP. Effectiveness was similar for ICU admission and need of mechanical ventilation, as well as by RSV subgroup. Children born pre-term or with birthweight <2500 g showed lower PP effectiveness, of about 70%.
Interpretation: Population-level nirsevimab immunoprophylaxis in children born or entering their first RSV season was very effective in preventing RSV hospitalisations, ICU admission and mechanical ventilation, with reduced but still high effectiveness for pre-term and low-birthweight children.
Funding: Institute of Health Carlos III (own budget).
Declaration of Interest: Matilde Zornoza Moreno has received research grants, travel grants and speaking fees from Sanofi and Astra-Zeneca. All other authors declare no conflict of interest.
Ethical Approval: Ethical approval was granted by the Medicines Research Ethics Regional Committee of the Community of Madrid (EV_NIRSEV_V2). The full study protocol is available in Spanish.
Nuñez, Olivier and Olmedo, Carmen and Moreno-Perez, David and Lorusso, Nicola and Fernández Martínez, Sergio and Pastor Villalba, Pedro Eliseo and Gutierrez, Ángeles and Alonso-García, Marcos and Latasa, Pello and Sancho, Rosa and Mendioroz Peña, Jacobo and Martinez-Marcos, Montse and Muñoz Platón, Enriqueta and García Rivera, María Victoria and Pérez-Martínez, Olaia and Álvarez-Gil, Rosa María and Rivas Wagner, Eva and López Gonzalez-Coviella, Nieves and Zornoza, Matilde and Barranco, M. Isabel and Pacheco Martínez, María del Carmen and Álvarez Río, Virginia and Fiol Jaume, Miguel and Morey Arance, Roxana and Adiego Sancho, Begoña and Mendez Diaz, Manuel and Batalla, Noa and Andreu, Cristina and Castilla, Jesús and Garcia Cenoz, Manuel and Fernández Ibáñez, Ana and Huerta Huerta, Marta and Ibañez, Ana Carmen and Berradre Sáenz, Belén and Lamas, Joaquín and Hermoso, Luisa and Casado Cobo, Susana and Galán Cuesta, Manuel and Montenegro, Sara and Domínguez, María and Jarrin, Inma and Limia, Aurora and Pastor-Barriuso, Roberto and Monge, Susana, Nirsevimab Effectiveness Against Rsv Hospital Admission in Children Under 1 Year of Age: A Spanish Population-Based Case Control Study. Available at SSRN: https://ssrn.com/abstract=4925473
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