Effectiveness and impact of universal prophylaxis with nirsevimab in infants against hospitalisation for respiratory syncytial virus in Galicia, Spain: initial results of a population-based longitudinal study

Read article: Submitted by: Samantha Bannister and Nan Vasilunas, Women’s and Children’s Hospital, Adelaide, 15/9/24.Background:Respiratory syncytial virus (RSV) causes lower respiratory tract infections (LRTI) in young children and accounts for more than 75% of hospital admissions in children under 5 years of age in Spain. Nirsevimab is a long-acting monoclonal antibody against RSV. In a phase 3 clinical trial assessing efficacy in infants born at a gestational age of 35 weeks or more, nirsevimab achieved 76.8% efficacy against hospitalisation for RSV-associated LRTI out to 150 days after administration (MELODY trial). A subsequent pragmatic European trial including over 8000 infants born at a gestational age of 29 weeks or more, nirsevimab had 83% effectiveness against hospitalisation for RSV-associated LRTI (HARMONIE trial). NIRSE-GAL is the first population-based longitudinal study to assess the real-world efficacy of nirsevimab in a high-prevalence population in the northwest region of Spain. This interim analysis includes more than 10,000 infants comprising both a birth cohort of infants (seasonal group) born between 25/9/23 – 15/12/23 and infants younger than 6 months of age (catch-up group) who received catch-up vaccination during this same period, with follow-up data to 31/12/23. Findings:91.7% of the 10,259 eligible infants in the seasonal and catch-up groups received nirsevimab. Nirsevimab effectiveness was 82% (95% CI 65.6-90.2) against hospitalisation for RSV-associated LRTI and 86.9% (95% CI 69.1-94.2) against RSV-associated LRTI requiring oxygen support. It also had 69.2% (95% CI 55.9-78.0) effectiveness against all-cause LRTI hospitalisations and 66.2% (95% CI 56.0-73.7) against all-cause hospitalisations. Hospitalisations for RSV-associated LRTI were reduced by 89.8% (IQR 87.5-90.3) compared to the previous five RSV seasons (excluding the COVID-19 pandemic period) and 25 infants (IQR 24-32) needed to be immunised to prevent one RSV-related URTI hospitalisation.Take home message:In this prospective population-based study, universal administration of nirsevimab prophylaxis to infants under 6 months of age during RSV season substantially reduced hospitalisation and need for oxygen support due to RSV-associated LRTI over a 3-month period. The high uptake of nirsevimab in the target population reflects a rapid and effective immunisation campaign.