Most infant immunization data stops at one season — making it genuinely difficult to judge whether protective effects persist or wane. This population-based study from Galicia, Spain now offers two consecutive seasons of real-world evidence on nirsevimab, the long-acting monoclonal antibody targeting RSV, closing a critical gap that pediatric clinicians and health planners have been waiting to fill.
The NIRSE-GAL study enrolled all infants eligible for nirsevimab under Galicia's universal prophylaxis campaign launched in 2023–24 and tracked outcomes through the end of the 2024–25 RSV season — roughly 18 months of follow-up. Primary endpoints centered on RSV-related lower respiratory tract infection (LRTI) hospitalization, with secondary endpoints spanning all-cause hospitalizations, bronchiolitis, pneumonia admissions, and a range of primary care outcomes including wheezing, otitis media, and acute bronchitis. Incidence rates were benchmarked against six pre-pandemic historical seasons (2017–18 through 2022–23) using Poisson regression models adjusted for RSV seasonality, lending statistical rigor to what is inherently an observational design.
The importance of a two-season window cannot be overstated. Nirsevimab's half-life is approximately 70–80 days, meaning direct antibody protection does not persist into a second season. Any sustained reduction in hospitalization rates in year two would point toward indirect population-level effects — herd protection or reduced viral transmission — rather than individual-level immunity. That mechanistic question distinguishes this study from earlier single-season effectiveness reports from France, the UK, and Luxembourg, which collectively established strong first-season efficacy in the range of 70–80% reduction in RSV hospitalizations. Whether those gains compound, attenuate, or plateau across successive seasons has direct implications for national immunization cost-effectiveness models. As a prospective observational study, confounding by healthcare-seeking behavior and surveillance intensity remains a limitation, but the population-wide enrollment in a defined geographic region substantially strengthens causal inference compared to hospital-only registries.