Respiratory syncytial virus has long been the leading infectious cause of infant hospitalization in high-income countries, yet until recently no broadly deployed immunoprophylaxis existed for healthy full-term newborns. The arrival of nirsevimab — a long-acting monoclonal antibody — raised hopes that RSV's seasonal pediatric burden could finally be blunted at a population level. Belgium's first full prevention season now provides some of the most compelling real-world evidence yet that those hopes were justified.
Drawing on two parallel national surveillance networks spanning 31 hospitals and covering nearly half of all Belgian pediatric beds, investigators compared RSV hospitalization rates between the 2023–2024 pre-prevention season and the 2024–2025 rollout season, when nirsevimab was offered to infants under six months of age entering their first RSV winter. The headline figures are striking: a 39% reduction in overall pediatric RSV-related hospitalizations and a 57% drop in RSV-linked intensive care admissions. Critically, the declines were concentrated almost entirely in the under-six-month age group — precisely the cohort targeted by nirsevimab — with older children showing no meaningful change, which strongly implies a causal rather than coincidental seasonal effect. Hospitalized cases in 2024–2025 also had shorter stays and required significantly less non-invasive ventilatory and nutritional support, pointing to reduced disease severity among breakthrough cases.
This Belgian dataset is noteworthy for several reasons. Unlike efficacy data from controlled trials, national surveillance captures real-world deployment imperfections — variable uptake, cold-chain issues, timing relative to season onset — meaning the observed reductions likely underestimate what optimal coverage could achieve. The concurrent availability of a maternal preF vaccine complicates clean attribution to nirsevimab alone, though the infant antibody dominated rollout in this period. Single-season comparisons also carry inherent uncertainty: RSV season severity varies year to year, and 2023–2024 may not represent a perfect baseline. Nevertheless, the age-specificity of the effect is a particularly robust internal control. Taken together with similar signals from France, Spain, and Luxembourg, this study adds substantial weight to what is becoming a consistent pattern: nirsevimab is delivering meaningful, measurable population-level protection against one of infant medicine's most burdensome pathogens.