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00925njm 22002777a 4500 dc Nuñez, Olivier author Olmedo, Carmen author Moreno-Perez, David author Lorusso, Nicola author Fernández Martínez, Sergio author Pastor Villalba, Pedro Eliseo author Gutierrez, Ángeles author Alonso Garcia, Marcos author Latasa, Pello author Sancho, Rosa author Mendioroz, Jacobo author Martinez-Marcos, Montserrat author Muñoz Platón, Enriqueta author García Rivera, María Victoria author Pérez-Martínez, Olaia author Álvarez-Gil, Rosa author Rivas Wagner, Eva author López González-Coviella, Nieves author Zornoza, Matilde author Barranco Boada, María Isabel author Pacheco, M Del Carmen author Álvarez Río, Virginia author Fiol Jaume, Miguel author Morey Arance, Roxana author Adiego Sancho, Begoña author Mendez Diaz, Manuel author Batalla, Noa author Andreu, Cristina author Castilla, Jesús author Garcia-Cenoz, Manuel author Fernández Ibáñez, Ana author Huerta Huerta, Marta author Ibáñez Pérez, Ana Carmen author Berradre-Sáenz, Belén author Lamas, Joaquín author Hermoso, Luisa author Casado Cobo, Susana author Galán Cuesta, Manuel author Montenegro, Sara author Domínguez, María author Jarrin Vera, Inmaculada author Limia, Aurora author Pastor-Barriuso, Roberto author Monge Corella, Susana author Nirsevimab Effectiveness Study Collaborators author 2025-02 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.AimWe 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, we selected four population density controls born in the same province and date (±2 days). 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. Results: We included 406 cases and 1,623 controls in catch-up and 546 cases and 2,182 controls in at-birth immunisation studies. Effectiveness in preventing RSV hospitalisations for catch-up immunisation was 71% (95% confidence interval (CI): 65-76) by ITT and 80% (95% CI: 75-84) PP. Effectiveness for at-birth immunisation was 78% (95% CI: 73-82) by ITT and 83% (95% CI: 79-87) PP. Effectiveness was similar for ICU admission, need of mechanical ventilation, and RSV viral subgroups A and B. Children born pre-term or with birthweight < 2,500 g showed lower PP effectiveness of 60-70%. Conclusions: Population-level nirsevimab immunoprophylaxis in children in 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. Núñez O, Olmedo C, Moreno-Perez D, Lorusso N, Fernández Martínez S, Pastor Villalba PE, Gutierrez Á, Alonso Garcia M, Latasa P, Sancho R, Mendioroz J, Martinez-Marcos M, Muñoz Platón E, García Rivera MV, Pérez-Martinez O, Álvarez-Gil R, Rivas Wagner E, López Gonzalez-Coviella N, Zornoza M, Barranco MI, Pacheco MDC, Álvarez Río V, Fiol Jaume M, Morey Arance R, Adiego Sancho B, Mendez Diaz M, Batalla N, Andreu C, Castilla J, García Cenoz M, Fernández Ibáñez A, Huerta Huerta M, Ibáñez Pérez AC, Berradre Sáenz B, Lamas J, Hermoso L, Casado Cobo S, Galán Cuesta M, Montenegro S, Domínguez M, Jarrín I, Limia A, Pastor-Barriuso R, Monge S; Nirsevimab Effectiveness Study Collaborators. Effectiveness of catch-up and at-birth nirsevimab immunisation against RSV hospital admission in the first year of life: a population-based case-control study, Spain, 2023/24 season. Euro Surveill. 2025 Feb;30(5):2400596. 1025-496X https://hdl.handle.net/20.500.12105/26330 39916606 10.2807/1560-7917.ES.2025.30.5.2400596 1560-7917 Euro surveillance : bulletin Européen sur les maladies transmissibles = European communicable disease bulletin Nirsevimab Effectiveness Hospitalisation Observational study Respiratory syncytial virus Effectiveness of catch-up and at-birth nirsevimab immunisation against RSV hospital admission in the first year of life: a population-based case-control study, Spain, 2023/24 season