Yellow fever spreads beyond its usual range in 2026, WHO reports

Yellow fever, a mosquito-borne viral disease of tropical Africa and the Americas, is circulating more widely in 2026 than its recent pattern would predict, WHO said in a global Disease Outbreak News update on 24 June 2026 [s1]. Between 1 January and 26 May 2026, six countries in the Americas reported 79 human infections alongside multiple epizootics, while three countries in Africa reported 16 confirmed human cases, with a further 32 suspected cases under investigation in five other countries [s1].

The concern in the notice is less the raw count — yellow fever is endemic across two continents — than where the cases are appearing. Since 2023, eight countries with no recent yellow fever activity have detected new cases, which WHO reads as a sign of viral circulation in areas that combine low vaccination coverage with limited surveillance [s1].

The disease, and why it matters

Globally, WHO estimates yellow fever causes between 67,000 and 173,000 severe cases each year and 31,000 to 82,000 deaths [s1]. Most infections are asymptomatic or produce a mild febrile illness, but about 15% of cases progress to a severe form marked by jaundice, haemorrhage and multi-organ failure; among those who reach that stage, case fatality can approach 50% within seven to ten days [s1]. The incubation period is typically three to six days [s1]. There is no specific antiviral treatment, which is why the disease is managed almost entirely through vaccination and mosquito control rather than at the bedside.

Africa: a coverage gap, not a mystery

Twenty-seven countries — 26 in WHO's African Region and one in the Eastern Mediterranean Region — are classified as high-risk under the Eliminate Yellow fever Epidemics (EYE) strategy [s1]. Of those, 26 have introduced the vaccine into routine immunisation, yet average coverage across the African Region was just 65% in 2024, below the 80% or higher the strategy sets for populations in at-risk areas [s1]. The January-to-May 2026 confirmed cases came from Burkina Faso, the Central African Republic and Cameroon, and the five countries with suspected cases under investigation were Angola, Côte d'Ivoire, Gabon, Ghana and Nigeria [s1]. In 2025, the region recorded two outbreaks, in Angola and the Central African Republic [s1].

WHO's assessment returns repeatedly to immunity: most human cases reported during 2025 and 2026 had no history of yellow fever vaccination [s1]. A single dose confers durable protection, so recurrence in under-vaccinated communities reads as a programmatic gap rather than a change in the pathogen — the same immunity shortfalls that have stalled childhood vaccination worldwide.

The Americas: a surge that has carried into 2026

The 2026 Americas figure follows a sharp 2025. The Pan American Health Organization counted 346 confirmed cases and 143 deaths across seven countries in 2025 — a case fatality rate of 41%, and 5.6 times the 61 cases recorded in 2024 [s2]. That surge, tracked in continuing alerts across the region in 2026, included transmission in lower-risk zones outside the Amazon basin [s2]. WHO's June update frames the detection of cases in previously unaffected areas as the element that most changes the risk picture, because it points toward populations with little immunity [s1].

Response, and what to watch

The global emergency stockpile of yellow fever vaccine, coordinated by the International Coordinating Group, is available to all countries for outbreak response and preventive campaigns [s1]. WHO advises affected countries to sustain epidemiological, epizootic and entomological surveillance and to reach and hold vaccination coverage above 80% in at-risk populations, supported by a strategic reserve that lets routine immunisation continue while outbreaks are contained [s1].

The variable WHO flags is ecological. Transmission is shaped by rainfall, temperature and mosquito abundance, and the outbreaks reported from October 2025 through May 2026 were broadly consistent with seasonal patterns or with immunisation gaps [s1]. The exception — spillover into a low-immunity urban setting where competent Aedes mosquitoes are present — is the scenario that would turn scattered rural infections into a large outbreak, and it is the one the agency is watching for [s1].

Sources

• Disease Outbreak News: Yellow fever - Global — World Health Organization, 24 June 2026
• Epidemiological Alert: Yellow fever in the Americas Region — 13 March 2026 — Pan American Health Organization / World Health Organization, 13 March 2026