[Nirsevimab immunization effectiveness against respiratory syncytial virus hospitalization in newborns: two season of use in Navarre, Spain].

Background: Respiratory syncytial virus (RSV) is the leading cause of infant hospitalisation. In 2022, nirsevimab was approved in the European Union to prevent severe respiratory disease due to RSV during the first year of life. Our aim is to evaluate the effectiveness of nirsevimab immunoprophylaxis in new-borns for preventing RSV -related hospitalisations in Navarre, Spain, during its first two seasons of use.

Methods: Nirsevimab was offered free of charge to infants born from October to December 2023 and from September to December 2024. Each cohort was followed until February of the following year. Cases were infants hospitalised for PCR-confirmed RSV infection. Cox regression was used to estimate the hazard ratio of hospitalisation for immunised versus non-immunised children.

Results: Nirsevimab was offered to 2,699 new-borns; of them, 2,541 (94.1%) received nirsevimab. In the 2023-2024 season, 17 RSV-related hospitalisations were recorded and 24 in the 2024-2025 season. The average risk of RSV hospitalisation was 7.6% in non-immunised new-borns versus 1.1% in immunised ones. Overall, effectiveness of nirsevimab was 79.5% (95% CI: 59.2 - 89.7), with estimates of 89.9% in 2023-2024 and 52.8% in 2024-2025, with no significant differences between seasons (p=0.055). On average, one RSV hospitalisation was prevented per 22.6 immunised infants.

Conclusions: Nirsevimab immunoprophylaxis substantially reduces RSV hospitalisations, helping ease paediatric hospital burden. However, as some immunised infants were still hospitalised, additional preventive measures remain necessary.