Respiratory syncytial virus kills more infants globally than any pathogen except malaria, yet until recently no broadly deployable immunization existed for newborns. Chile's decision to roll out nirsevimab — a long-acting monoclonal antibody, not a traditional vaccine — nationwide in 2024 created a rare natural experiment: the first Southern Hemisphere country to attempt universal infant RSV protection at scale, generating real-world outcome data that researchers could rigorously evaluate.
Using the Augmented Synthetic Control Method applied to Chile's national health registry from 2019 through 2024, investigators modeled what the 2024 RSV season would have looked like without the program, then compared that counterfactual to observed outcomes. Across the April–November 2024 window, nirsevimab immunization was associated with roughly 25,632 fewer ICU bed-days, approximately 59,072 fewer general hospital bed-days, around 25,620 fewer outpatient medical encounters, and an estimated 20,430 fewer days of maternal medical leave. The program generated a net economic benefit of approximately USD 23.5 million after deducting immunization costs, with the public sector accounting for USD 15.5 million of that figure. Notably, the catch-up cohort — infants born October 2023 through March 2024 who received nirsevimab before their first RSV season — contributed over 53% of total savings, underscoring that reaching older infants still in their first year is economically critical.
This study represents one of the most methodologically rigorous real-world assessments of nirsevimab to date. The ASCM approach is considerably more robust than simple pre-post comparisons, though it still cannot fully control for concurrent interventions or secular trends in RSV epidemiology — particularly relevant given post-pandemic RSV seasonality shifts globally. The finding that the catch-up cohort drove the majority of benefit challenges program designers who might prioritize only newborns for resource efficiency. For health systems weighing nirsevimab adoption, Chile's data provide a compelling cost-benefit template, though transferability depends heavily on baseline RSV hospitalization rates, healthcare costs, and immunization infrastructure. Overall, this is a confirmatory and practically significant contribution rather than a paradigm shift — it validates trial-era efficacy estimates in a real national program.