WASHINGTON (CMC) – The Pan American Health Organization (PAHO) on Tuesday issued an epidemiological alert regarding yellow fever in the Americas following the recent increase in confirmed human cases in several countries in the region and a change in the geographic distribution of the disease.

Yellow fever is caused by a virus that is transmitted to humans through the bite of infected mosquitoes. The virus is a flavivirus, which is the same group of viruses that cause dengue, West Nile, and other diseases.

PAHO said that this increase has been observed over the last months of 2024 and the first weeks of 2025.

It said in total, 61 cases of yellow fever were confirmed in 2024, with 30 resulting in death. This number exceeds the 58 reported cases of yellow fever, including 28 deaths, between 2022 and 2023 in Bolivia, Brazil, Colombia, and Peru. In January of this year, 17 additional cases were reported, with seven deaths.

PAHO said that while in 2024, cases were mainly concentrated in the Amazon region of Bolivia, Brazil, Colombia, Guyana, and Peru, in 2025, the disease has begun to spread to areas outside this zone, particularly to the state of São Paulo, Brazil, and the department of Tolima, Colombia. Peru has also reported a fatal case.

PAHO warns that other countries could also be affected.

Yellow fever is a serious viral disease that can be fatal, especially in its more severe form. PAHO said this increase in cases highlights the urgent need to intensify efforts to prevent the spread of the virus, strengthen clinical management, with an emphasis on early detection and treatment of severe cases, and improve epidemiological surveillance in high-risk areas.

PAHO is reminding countries that vaccination remains one of the most effective tools for preventing and controlling yellow fever.

In 2024, most of the cases reported were in people who had not been vaccinated. In this regard, PAHO urges countries to continue strengthening their vaccination programs and to take appropriate measures to inform and protect travellers heading to high-risk areas.

Given the changes in the patterns of virus transmission, PAHO stresses the need to maintain active surveillance, particularly in areas near affected zones, to detect any suspected cases early and ensure timely isolation and treatment.

Among the key recommendations are to rapidly detect any suspected cases, even in areas not traditionally affected, and ensure that at least 95 per cent of people in high-risk areas are vaccinated.

PAHO is also recommending that states conduct virological diagnostics via PCR during the first seven to 10 days of illness, or ELISA IgM in the convalescent phase, with careful interpretation due to cross-reactivity in areas with other circulating flaviviruses.

In addition, there is a need to strengthen early detection and specialised monitoring of severe patients as a key measure to save lives and review and update the national and subnational vaccine inventory to plan a rapid emergency response.

Since 1970, yellow fever has reemerged as a public health threat in the Americas. The disease is endemic in 13 countries and territories in the region, causing outbreaks and deaths. In 2014, the virus spread beyond the Amazon basin. Some attribute this spread to changes in the interaction between monkeys, mosquitoes, and humans.

PAHO said it continues to closely monitor the situation to provide the most up-to-date information and best practices to mitigate the impact of the disease, ensure a coordinated response among countries in the region, and offer technical and strategic support for the prevention and control of yellow fever.