Impact of nirsevimab immunization on RSV infections attended in the pediatric emergency department: First results in a tertiary hospital in Madrid

Introduction: Human respiratory syncytial virus (RSV) is the most commonly identified virus associated with lower respiratory tract infections. The monoclonal antibody nirsevimab immunization campaign began in our country in October 2023. Methods: This study was conducted in the Pediatric Emergency Department (PED) of a tertiary care center in Madrid, Spain. The aim was to compare PED visits of patients eligible for immunization with nirsevimab who attended between weeks 40 and 52 of 2022 and 2023 and who had a confirmed diagnosis of RSV infection. Results: During the study period, 264 out of 765 patients with confirmed RSV infection who attended the PED were eligible for immunization with nirsevimab and were selected for our analysis. The PED attendance was 80.3% in 2022 and 19.7% in 2023. The number of RSV-positive cases increased from week 42 in both analyzed periods, with a peak of maximum incidence between weeks 46 and 48. In 2022, the morphology of the case curve in the group of children eligible for immunization was similar to the overall curve. However, in 2023, we did not observe a similar increase in cases among patients eligible for immunization. Conclusion: Immunization with nirsevimab during the 2023 RSV epidemic season had a beneficial effect, reducing the number of PED consultations for RSV infection.