Yellow Fever Fatality Rate Increases 20%

Over the past few centuries, yellow fever outbreaks have led to numerous cases and related fatalities. While the World Health Organization says most severe yellow fever (YF) cases have a fatality rate of around 39%, recent data from South America indicates an unfortunate increase in deaths.

Since 2024, the Republic of Colombia has faced an active YF outbreak. As of February 21, 2025, 40 cases have been confirmed (17 in 2025), with 19 related deaths, representing a case fatality rate (CFR) of 47%. 

As of mid-March 2025, Columbia's YF outbreak remains active in the departments of Tolima, Caquetá, Huila, Nariño, Putumayo, and Vaupés.

Throughout South America, between the last months of 2024 and the first weeks of 2025, the Pan American Health Organization (PAHO) reported 61 cases of YF, 30 of which resulted in death, for a CFR of 49%.

These increased CFRs are similar to those reported between 2022 and 2023 in Bolivia, Brazil, Colombia, and Peru.

According to the PAHO, one factor in the approximate 20% increase in CFR is the expanded geography of YF-transmitting mosquitos.

The PAHO recently issued an epidemiological alert regarding a change in the geographic distribution of the YF disease. All cases had a history of exposure in areas at risk for the YF virus, such as wild and wooded areas, in the context of work activities that included agriculture, and had no documented vaccination history.

Another factor is that these new areas are unprepared to diagnose and treat YF infection before the infection worsens.

"A single case of yellow fever can constitute an outbreak, as it can spread rapidly. Therefore, one of the ways this disease is monitored is by observing how it behaves in nature. When non-human primates begin to become infected and die, this alerts us to the activity and potential spread of the virus, and allows us to anticipate epidemics in humans," explained Jairo Méndez Rico, Regional Advisor for Viral Diseases at PAHO's Department of Health Emergencies in a media release on March 11, 2025.

Various studies have determined that mosquito-borne diseases such as YF, chikungunya, dengue, and malaria are expanding their range and re-emerging in areas where they had subsided for decades. The extent to which climate change influences the population at risk of mosquito-borne diseases varies across altitudes.

In March 2025, the PAHO and the U.S. CDC continue encouraging everyone visiting a yellow fever endemic area to be protected by vaccination. YF vaccines have been found safe and effective and are commercially available at certified travel clinics and pharmacies in the United States.