Yellow fever is appearing outside the Amazon, PAHO warns, with 44% case fatality
The regional health agency counted 34 confirmed cases and 15 deaths in the first seven weeks of 2026, after 2025 brought the highest South American case total in over two decades.
The Pan American Health Organization issued a new epidemiological alert on 13 March warning that yellow fever transmission in South America remains elevated into 2026, after the region recorded its highest annual case count in decades the year before [s1].
The numbers behind the alert
Between epidemiological week 1 and week 53 of 2025, PAHO recorded 346 confirmed human yellow fever cases across seven countries in the Americas, including 143 deaths — a case fatality rate of 41% [s1]. That case total was 5.6 times higher than 2024, when 61 confirmed cases were reported [s1]. Measured against the historical record going back to 1960, 2025's total is high but not unprecedented — comparable to other elevated years such as 1966 (304 cases), 1998 (280 cases), and 2003 (243 cases) [s1].
The 2025 cases broke down by country as follows: Brazil, 120 cases and 48 deaths; Colombia, 125 cases and 46 deaths; Peru, 49 cases and 19 deaths; Venezuela, 32 cases and 19 deaths; Bolivia, 8 cases and 2 deaths; Ecuador, 11 cases and 8 deaths; and Guyana, 1 death [s1].
Transmission has continued into the new year. Between week 1 and week 7 of 2026, PAHO confirmed 34 human cases and 15 deaths across four countries — a case fatality rate of 44% [s1]. Colombia carried the largest share, with 25 cases including 13 deaths; Venezuela reported 6 cases and 1 death; Peru reported 2 cases; and Bolivia reported 1 death [s1].
Why PAHO is flagging geography, not just case counts
The alert's central concern is not only that cases are elevated, but where they are appearing. Since September 2024, yellow fever cases have been detected in areas with no history of transmission, including locations outside the Amazon basin [s1]. In Colombia, cases in 2026 have concentrated in the department of Tolima, and PAHO specifically notes detections in mountain forest areas — the Bosque de Galilea Regional Natural Park, on the western slope of the Eastern Cordillera connecting to the Sumapaz páramo [s1]. In Venezuela, 2026 cases have centered on the states of Aragua, Barinas, Lara, and Monagas [s1].
This matters because yellow fever's usual transmission pattern in South America is sylvatic — spread through a jungle cycle involving mosquitoes and nonhuman primates in forested areas, geographically distinct from the urban transmission cycle carried by Aedes aegypti mosquitoes that drove historic yellow fever epidemics in cities. PAHO's alert notes that the expansion of sylvatic transmission toward forest edges near urban centers raises the risk of the virus crossing into that urban cycle [s1] — a scenario the region has avoided for decades but which vaccination-coverage gaps and geographic spread both make more plausible.
Brazil's 2025 cases illustrate the pattern: most were concentrated in 29 municipalities across six regions of São Paulo state, itself a departure from the disease's traditional Amazonian territory [s1]. Venezuela's 2025 outbreak similarly reached 22 parishes not previously classified as risk areas, across four states, with the states bordering the expanding "San Camilo" enzootic wave — Apure, Táchira, Lara, Mérida, Barinas, Portuguesa, Cojedes, and Guárico — accounting for 61% of that year's national case total [s1].
The standing risk classification
PAHO's formal public health risk assessment for yellow fever in the Americas, first issued in February 2025 and updated in May 2025, classifies the regional risk as "High" [s1]. The March 2026 alert does not revise that classification but reaffirms the factors behind it: rising case counts, high case fatality rates, and spread into areas with no prior record of viral circulation [s1].
What PAHO is recommending
The alert calls for member states to maintain at least 95% vaccination coverage among populations in at-risk areas, and to ensure travelers heading to affected regions are vaccinated at least 10 days before arrival — the interval needed for protective immunity to develop [s1]. A single dose of yellow fever vaccine provides lifelong protection, according to PAHO [s1]. The organization is also pressing for strengthened surveillance to catch geographic spread earlier, and for improved clinical management focused on early detection and treatment of severe cases, given the high case fatality rate among those who do develop symptomatic illness [s1].
What to watch
The core open question is whether 2026's case trajectory — 34 cases and 15 deaths in just seven weeks — will track toward or beyond 2025's elevated total, and whether the geographic expansion documented in Colombia and Venezuela continues to push transmission closer to dense urban populations where an Aedes aegypti-driven outbreak could unfold very differently from the rural, sylvatic pattern PAHO has tracked for most of the past two years.
Sources
- Epidemiological Alert: Yellow fever in the Americas Region — 13 March 2026 — Pan American Health Organization / World Health Organization, 13 March 2026
Sources
- Epidemiological Alert: Yellow fever in the Americas Region — 13 March 2026 — Pan American Health Organization / World Health Organization , March 13, 2026
WHO closes 2025 with 96 countries measles-free and a warning about new vaccine financing
The year-end immunization summary reports 88% fewer measles deaths since 2000, malaria vaccine in 24 African countries, and 25 years without wild polio in the Western Pacific.
Bolivia's 597 measles cases led South America in the region's worst year since 2020
PAHO counted 14,891 confirmed cases across the Americas in 2025, a 32-fold rise on 2024. Six South American countries reported cases, and only a third of the region's countries reach the coverage that stops outbreaks.
Michigan reports 6,065 cyclosporiasis cases in an unusually large summer outbreak
An interim report in NEJM Evidence links the parasitic outbreak to iceberg lettuce and one restaurant chain, with cases spread across most of the state.
Meningococcal cases tracked Umrah as pilgrim vaccination compliance fell to 54%
WHO recorded serogroup W135 disease in pilgrims returning from Saudi Arabia in early 2025. A modelling study estimates high vaccination coverage would cut expected cases by more than 80%.