Yellow fever activity in the Americas Region has intensified over the past two years, with 2025 recording the highest annual number of confirmed cases since 2020 (Figure 1). This resurgence follows major outbreaks reported in 2017 and 2018 and is notable for cases in areas without a historical record of transmission, including outside of the Amazon Region. Preventing further spread of the virus requires maintaining strong surveillance systems, sustaining high vaccination coverage in populations living in high-risk areas, and ensuring a timely detection and coordinated response.
Between epidemiological week (EW) 1 and EW 53 of 2025, a total of 346 confirmed human cases of yellow fever and 143 deaths (case fatality rate [CFR] 41%) were reported across seven countries of the Americas Region. This represents a 5.6-fold increase compared with 2024, when 61 confirmed cases were reported. Countries reporting cases in 2025 included Bolivia, Bolivarian Republic of Venezuela, Brazil, Colombia, Ecuador, Guyana, and Peru. In the first seven weeks of 2026, 34 confirmed human cases including 15 deaths (CFR 44%) were reported in four countries: Bolivia, Colombia, Peru, and Venezuela. The continued occurrence of cases in the first weeks of 2026 indicates ongoing transmission and highlights the need for sustained surveillance across the Region.
Beyond the overall increase in cases, recent epidemiological patterns reveal the occurrence of yellow fever cases into areas not historically considered at risk. Since 2024, cases have been observed in a wider geographical area than in previous years, including high-elevation forest areas associated with the Bosque de Galilea region in Tolima, Colombia. In Brazil, most cases reported in 2025 occurred across 29 municipalities in São Paulo State; in Venezuela, cases have also expanded geographically, spanning multiple states and areas previously not classified as risk zones.
In parallel, epizootic events in nonhuman primates have been reported across several countries, particularly in Bolivia, Bolivarian Republic of Venezuela, Brazil, and Colombia. In multiple settings, primate mortality preceded or coincided with human cases. In ecological perspective, these events across an extended area of the continent highlight ongoing sylvatic transmission and spillover at forest edges, where human activities intersect with biodiverse ecosystems. These environments provide ideal conditions for canopy-dwelling mosquitoes to transmit the virus from nonhuman reservoirs to human populations.
Read original in English: https://doi.org/10.26633/ RPSP.2026.51