Yellow fever - Global

Yellow fever remains a significant public health threat in regions with historical transmission, particularly in parts of Africa and South America. Although the virus is maintained primarily through mosquito–primate transmission cycles, periodic spillover into human populations continues to occur, especially in forested and rural environments. While competent vectors are widely present and ecological and peri‑urban habitats are expanding, the potential for spread into new areas, including urban centres, remains substantial, particularly where population immunity is low. Outbreak risk is further amplified by population movement, fragile health systems, and gaps in routine vaccination.

Although most affected countries have established surveillance systems, insecurity, limited healthcare access, and delayed clinical presentation frequently hinder outbreak investigations and timely treatment. While early symptoms resemble other endemic diseases and laboratory capacity is often constrained, delays in diagnosis contribute to under‑reporting, especially in remote areas. As a result, the true burden of yellow fever is likely underestimated.

Unvaccinated individuals living in rural or forest‑edge communities remain the most exposed, while urban and peri‑urban populations in newly affected areas may also be at risk when competent vectors are present and immunity gaps persist. Travellers who are not vaccinated and who move into high‑transmission regions similarly face increased risk.

Yellow fever introduction into regions where Aedes aegypti is established remains possible through viremic travellers arriving from high‑transmission settings. Although no urban transmission has been documented in 2025-2026 in high‑risk countries, limited vector‑ surveillance and control capacity could facilitate spread if the virus were introduced in an insufficiently immunized population. While a sylvatic cycle has not been established in regions without prior yellow fever circulation, ecological suitability, cross‑border movement, the presence of non‑human primates, and gaps in immunity and surveillance continue to create vulnerability in several African countries classified as moderate risk. In countries lacking competent vectors, imported cases may occur, although onward transmission is unlikely; in these settings, the primary challenge is timely clinical recognition.

Vaccination remains the strongest determinant of risk. While vaccinated populations are well protected, unvaccinated individuals in at‑risk areas face the highest likelihood of infection and severe disease. Even in areas without documented circulation, limited surveillance means that undetected transmission cannot be entirely excluded.

As of 17 June 2026, WHO assesses the risk of YF transmission to be low at the global level and moderate in regions with historical transmission, specifically the WHO African Region and the WHO Region of the Americas. Further details on the WHO risk assessment are available here.