Abstract: Background and Objectives: Spain was among the first countries to implement universal prophylaxis for respiratory syncytial virus (RSV) using nirsevimab, leading to a 60%?80% reduction in bronchiolitis hospital admissions. The main objective of this study was to describe the characteristics of infants admitted with bronchiolitis after RSV prophylaxis had been established. Methods: A retrospective study was conducted in 15 Spanish pediatric emergency departments during the 2023?2024 RSV season. We reviewed medical records coded as ?acute bronchiolitis? of infants aged less than 12 months who were hospitalized. Results: A total of 947 infants with bronchiolitis were included. Of these, 198 (20.9%) were ineligible for immunization because they were older than 6 months and had no comorbidities. Among the 749 (79.1%) infants who met the criteria for immunization, 377 (50.3%) received nirsevimab, of whom 159 (42.1%) tested positive for RSV, while 372 (49.7%) were not immunized, with 322 (86.5%) testing positive for RSV (p < 0.001). Among infants who were not immunized, the most common reason for not receiving nirsevimab was family refusal. Among infants with RSV bronchiolitis, there were no significant differences in PICU admission rates or length of stay between those who were immunized and those who were not. Conclusion: During the first nirsevimab campaign, most bronchiolitis admissions involved healthy children over 6 months of age who were not included in the target population, eligible infants who were not immunized, and immunized infants with non-RSV bronchiolitis. Further studies are needed to analyze the characteristics of immunized patients who are admitted with RSV bronchiolitis.
