Health care utilization and cost implications of Chile's 2024 nirsevimab strategy for RSV prevention: a counterfactual analysis.

Background: In 2024, Chile became the first Southern Hemisphere country to implement a universal nirsevimab immunization strategy against respiratory syncytial virus (RSV). This study evaluates the real-world health impact and cost-effectiveness of the program, specifically comparing seasonal (born April-September 2024) and catch-up (born October 2023-March 2024) cohorts. Methods: We performed a retrospective analysis using Chilean nationwide health registry data. The Augmented Synthetic Control Method (ASCM) utilized 2019-2024 data to estimate a counterfactual scenario for April-November 2024. Outcomes included outpatient medical attentions, basic/intermediate hospital bed-days, intensive care unit (ICU) bed-days, and days of maternal medical leave, identified through diagnoses for lower respiratory tract infections (LRTIs) and related viral agents. Differences between observed and predicted events were used to calculate treatment effects and cost-benefit ratios based on national healthcare costs. Findings: Nirsevimab was associated with substantial reductions across all metrics: approximately 20,430 days of medical leave, 25,620 medical attentions, 59,072 basic/medium bed-days, and 25,632 ICU bed-days. These reductions translated into a net economic benefit after subtracting the cost of immunization, of USD 23.50 million, including USD 15.50 million in the public healthcare sector. The catch-up cohort accounted for 53.41% of total cost savings, and the seasonal cohort for 46.59%. Interpretation: Chile's nationwide nirsevimab rollout substantially reduced infant RSV morbidity and healthcare utilization. These real-world findings demonstrate that universal immunization with long-acting monoclonal antibodies can significantly decrease hospitalizations and health system burden, supporting broad prevention strategies that include infants born prior to the RSV season. Funding: This research was partially funded by Instituto Sistemas Complejos de Ingeniería (ISCI) and by an independent research grant from Sanofi-AstraZeneca received by the ISCI team.