Characterization of an Unusual Severe Yellow Fever Outbreak in the Colombian Andean Region.

Background: Yellow fever (YF) reemerged in Colombia's Andean region during 2024-2025, outside the country's historically recognized endemic areas. We characterized critically ill adults admitted during this outbreak and described clinical features, organ dysfunction, supportive care, and in-hospital outcomes. Methods: We conducted a prospective observational cohort study of consecutive adults with reverse transcription polymerase chain reaction (RT-PCR)-confirmed YF admitted to the national referral intensive care unit in Ibagué, Tolima, Colombia, from March through September 2025. Clinical, laboratory, complication, intervention, and outcome data were prospectively captured using a standardized case report form. Results: Sixty-one patients were analyzed; 22 died (36.1%). Non-survivors had lower platelet counts and fibrinogen and higher INR, aminotransferases, lactate dehydrogenase, and creatinine at ICU admission. Shock, acute kidney injury, acute respiratory distress syndrome, disseminated intravascular coagulation, and severe bleeding were more frequent among non-survivors. Prior YF vaccination was self-reported by 10 patients (16.4%). Conclusions: Severe YF in this Andean outbreak was characterized by hepatic-coagulation abnormalities and multisystem organ dysfunction. The occurrence of critical illness outside historically recognized endemic areas supports surveillance and vaccination strategies that can adapt to changing epidemiological risk.