José F. Barbas Del Buey ^1* Jesús Íñigo Martínez ² María Á. Gutiérrez Rodríguez ² Marcos Alonso García ² Amaya Sánchez Gómez ² María D. Lasheras Carbajo ² Susana Jiménez Bueno ² María D. Esteban Vasallo ² María A. López Zambrano ² Cristina Calvo Rey ^3,4,5 Manuel Sanchez Luna ^6,7 Marta Molina Olivas ² María A. Arce Arnáez ²

• ¹ FIIBAP Fundación para la Investigación e Innovación Biosanitaria de Atención Primaria, Madrid, Spain
• ² Dirección General de Salud Pública. Consejería de Sanidad de la Comunidad de Madrid, Madrid, Spain
• ³ Hospital Infantil La Paz, Madrid, Madrid, Spain
• ⁴ University Hospital La Paz Research Institute (IdiPAZ), Madrid, Madrid, Spain
• ⁵ Autonomous University of Madrid, Madrid, Madrid, Spain
• ⁶ Neonatology Division, University Hospital Gregorio Marañón, Madrid, Spain
• ⁷ Complutense University of Madrid, Madrid, Madrid, Spain

Introduction: Respiratory syncytial virus (RSV) infection is one of the main causes of morbidity and mortality from lower respiratory tract infections in children under 5 years of age worldwide. Given that, the objective of this study was estimate the effectiveness of nirsevimab (a singledose, long-acting, human recombinant monoclonal antibody against RSV) over time for the prevention of respiratory episodes treated at different levels of care.Methods: A prospective and dynamic population-based cohort study was performed including infants born between April 1 and December 31, 2023, in the Madrid region who resided there during the follow-up period from October 1, 2023, to February 29, 2024. Infants were considered immunized from the day after receiving one dose (50 or 100 mg) of nirsevimab or nonimmunized individuals if they did not receive any dose.Results: There were 4,100 episodes of primary care, 1,954 hospital emergencies, and 509 admissions, 82 of which required intensive care in the 33,859 participants analysed.The adjusted effectiveness of nirsevimab in preventing hospitalization due to RSV infection was 93.6% (95% CI: 89.7, 96.1) at 30 days and 87.6% (95% CI: 67.7 to 95.3) at 150 days. The NNTs number needed to treat to prevent one avoid additional hospitalization were 314.19 (95% CI: 306.22 to 327.99) at 30 days and 24.30 (95% CI: 22.31 to 31.61) at 150 days.The adjusted effectiveness of nirsevimab in avoiding admission to an intensive care unit was 94.4% (95% CI: 87.3 to 97.5) at 30 days and 92.1% (95% CI: 64.0, 98.3) at 90 days. The adjusted effectiveness of nirsevimab for avoiding primary care consultations and hospital emergency visits was lower.Discussion: Immunization with nirsevimab is an effective measure for reducing the burden of care related to RSV at all levels of care albeit it decreases throughout follow-up. At 150 days it remained high for preventing hospital admissions.Other articles already published have also demonstrated high effectiveness although with preliminary results, short follow-up periods and wide confidence intervals. None have detected a decrease in effectiveness over time.These results can be quite useful in individual infant prevention and in the design of immunization campaigns.