Population Impact of a hybrid strategy combining maternal RSV vaccination and nirsevimab immunization on lower respiratory tract infections in infants under 1 year in Bogota: a counterfactual analysis

Objective: To assess the impact of a hybrid RSV immunization strategy on hospitalizations, pediatric intensive care unit (ICU) admissions, outpatient visits, and mortality due to LRTI among infants under one year of age in Bogota. Methods: We conducted an ecological interrupted time-series study using weekly surveillance data from Bogota from EW 1 of 2023 to EW 24 of 2026 (181 weeks). Outcomes included weekly rates of all-cause viral LRTI-related general hospitalizations, pediatric ICU admissions, outpatient visits, and deaths among infants younger than one year. Segmented negative binomial regression models adjusted for secular trends, seasonality using Fourier terms, and autocorrelation were used to estimate changes associated with maternal RSVpreF vaccination and nirsevimab implementation. Counterfactual analyses were performed to estimate cases averted and relative risk reductions. Results: Compared with the same period in 2025, the 2026 LRTI hospitalization rate decreased significantly (rate ratio [RR] 0.66; 95% CI 0.64-0.68), as did pediatric ICU admissions (RR 0.78; 95% CI 0.73-0.85) and outpatient visits (RR 0.78; 95% CI 0.77-0.79). Interrupted time-series analyses identified a significant weekly decline in hospitalization trends following maternal RSVpreF introduction (3.9% per week; p=0.023) and a smaller but significant decline in ICU admissions (-2.8% per week; p=0.039). The cumulative relative reduction in hospitalizations was estimated at 47.1% (95% CI 13.9-70.4), corresponding to 7.605 hospitalizations averted over the post-intervention period (EW 47/2025-EW 24/2026). No statistically significant changes were observed for outpatient visits or mortality. Conclusions: Implementation of a hybrid RSV prevention strategy was associated with a substantial reduction in severe LRTI among infants during the first respiratory season following introduction in Bogota. These findings provide the first real-world population-level evidence from Latin America supporting hybrid RSV immunization as a feasible and potentially cost-effective strategy for reducing severe infant respiratory disease in middle-income settings. Keywords: Respiratory syncytial virus (RSV); Maternal RSV vaccination; Nirsevimab; Hybrid immunization strategy; Population impact; Lower respiratory tract infection (LRTI); Interrupted time series ( ITS); Bogota, Colombia.