Adverse Events Following Yellow Fever Vaccination in Older Adults in Colombia, 2024-2025: A National Pharmacovigilance Analysis.

Abstract

Introduction: Yellow fever re-emerged as a major public health threat in the Americas during 2024-2025, with increasing numbers of cases and deaths, predominantly among unvaccinated individuals. Although the live-attenuated 17D yellow fever vaccine is highly effective, adults aged ≥60 years have an increased risk of serious adverse events following immunization (AEFIs). We assessed the safety profile of yellow fever vaccination in Colombia, with particular emphasis on older adults, and contextualized the observed risks within an epidemic setting characterized by active transmission and high mortality. Methods: We conducted an observational, descriptive study using secondary data from Colombia's Expanded Program on Immunization and the national VigiFlow pharmacovigilance system. Individuals vaccinated against yellow fever between 1 October 2024 and 31 July 2025 were included. We analyzed reported AEFIs by seriousness, sex, and age group. We calculated rates per 100,000 vaccinated individuals with 95% confidence intervals and compared them between adults aged ≥60 years and younger individuals. We further stratified older adults into five-year age groups. Results: Among 3,915,966 vaccinated individuals, 425 AEFIs were reported in 233 unique cases. The overall AEFI reporting rate was 10.85 per 100,000 vaccinated individuals, and the serious AEFI reporting rate was 2.94 per 100,000. Adults aged ≥60 years had substantially higher overall AEFI reporting rates than those aged <60 years (54.05 versus 6.12 per 100,000, respectively), corresponding to a rate ratio of 8.8. Serious AEFI reporting rates were 11.90 and 1.96 per 100,000, respectively, corresponding to a rate ratio of 6.1. The highest serious AEFI reporting rate was observed among adults aged 80-84 years, at 49.82 per 100,000. Nine fatal outcomes were reported; following causality assessment, none was considered attributable to the vaccine. Conclusions: Reported AEFIs following yellow fever vaccination were more frequent among adults aged ≥60 years, with a particularly marked increase in the oldest age groups. Nevertheless, the absolute serious AEFI reporting rate remained low, at approximately 1.2 per 10,000 vaccinated older adults, and no reviewed fatal outcomes were attributed to vaccination. In settings of active yellow fever transmission, vaccination decisions for older adults should therefore be guided by individualized benefit-risk assessment rather than age alone, accompanied by careful screening, active pharmacovigilance, and clear risk communication. Vaccination remains the principal intervention for preventing yellow fever-related deaths.