## Understanding the Global Yellow Fever Situation Yellow fever, a serious viral illness spread by infected mosquitoes, continues to pose a public health challenge across specific regions of Africa and the Americas. Recent data from the World Health Organization (WHO) highlights continued transmission activity into 2026, following a noticeable increase in cases during 2025, particularly in the Americas. This ongoing situation underscores the critical need for robust prevention strategies, with vaccination remaining at the forefront of global efforts to control and eventually eliminate the disease. From January to May 2026, six countries in the Americas – Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela – collectively reported 79 confirmed human infections. These cases were accompanied by numerous 'epizootics,' a term used when the disease affects animals, particularly monkeys, signaling active circulation of the virus in jungle environments. Meanwhile, in Africa, sustained yellow fever activity was observed across 13 countries identified as high-risk under the global Eliminate Yellow Fever Epidemics (EYE) Strategy. During the same five-month period in 2026, three African nations – Burkina Faso, Central African Republic, and Cameroon – confirmed 16 human cases, with another 32 suspected cases under investigation in five other countries, including Angola, Côte d’Ivoire, Gabon, Ghana, and Nigeria. These occurrences emphasize the ongoing battle against this preventable disease. ## The Yellow Fever Virus and How It Spreads Yellow fever is caused by a virus transmitted primarily by day-biting mosquitoes, mainly species like *Aedes*, *Haemagogus*, and *Sabethes*. The disease is endemic to tropical and subtropical areas of Africa and South America. Its transmission typically occurs through three main cycles: the sylvatic (jungle) cycle, the intermediate cycle, and the urban cycle. The sylvatic cycle involves mosquitoes passing the virus between monkeys, with humans becoming infected when they enter these forested areas and are bitten by infected mosquitoes. The intermediate cycle involves semi-domestic mosquitoes infecting both monkeys and humans, usually in areas where forests and human settlements meet. The urban cycle, the most concerning for public health, occurs when *Aedes aegypti* mosquitoes, often found in densely populated areas, transmit the virus directly between humans, leading to large-scale outbreaks. The risk of yellow fever transmission is significantly influenced by several factors, including seasonal ecological conditions such as rainfall, temperature, and the abundance of mosquito populations. Human activities also play a crucial role; increased mobility, the expansion of urban areas into forested regions, and uneven vaccination coverage facilitate the virus's spread. Detecting cases in previously unaffected zones is particularly worrying as it suggests the virus has been introduced into new areas, heightening the risk of broader, potentially urban, outbreaks. Despite these challenges, no international spread to regions outside Africa and the Americas has been reported recently, yet the potential for global transmission remains due to expanding mosquito habitats, rapid urbanization, climate shifts, and extensive international travel. ## Recognizing the Symptoms and Disease Progression After an infected mosquito bite, the incubation period for yellow fever typically ranges from 3 to 6 days. Many individuals experience no symptoms at all, or only very mild ones that can be easily mistaken for other common viral infections. These initial mild symptoms often include fever, headache, muscle pain, nausea, and vomiting, usually resolving within a few days. However, about 15% of those infected will develop a more severe form of the disease. This critical phase is characterized by a recurrence of high fever, accompanied by jaundice (yellowing of the skin and eyes, giving the disease its name), bleeding (haemorrhage), and multi-organ failure, particularly affecting the liver and kidneys. Sadly, for those who develop this severe illness, the case fatality rate can be as high as 50% within 7 to 10 days. Diagnosing yellow fever can be complex because its symptoms overlap with other prevalent diseases like malaria, dengue fever, and viral hepatitis, which can lead to underreporting and delays in crucial treatment and outbreak response. Rapid laboratory confirmation is vital for effective management and control efforts. ## Vaccination: Our Primary Defense Vaccination stands as the single most effective tool for preventing and controlling yellow fever. The yellow fever vaccine is remarkably safe, affordable, and provides lifelong protection after just one dose. This makes it a cornerstone of the WHO's ambitious Eliminate Yellow Fever Epidemics (EYE) strategy, which aims to protect populations aged 9 months to 60 years in all high-risk countries. The strategy emphasizes both routine immunization programs for children and large-scale preventive vaccination campaigns to build strong community immunity. WHO and its partners have been actively supporting these immunization efforts. Between 2025 and May 2026, preventive mass vaccination campaigns successfully reached over 35 million people in high-risk and affected countries across Africa. Significant campaigns included reaching 15.2 million people in DR Congo, 1.6 million in Guinea-Bissau, 9.6 million in Niger, and 4.2 million in Uganda. Reactive campaigns, launched in response to outbreaks, also protected hundreds of thousands in Burkina Faso, Cameroon, Côte d’Ivoire, and Mali. For travelers heading to endemic areas, vaccination is strongly recommended to prevent infection and reduce the risk of international spread. The International Health Regulations (2005) allow countries to require proof of yellow fever vaccination, documented by an International Certificate of Vaccination or Prophylaxis (ICVP), which is accepted as valid for life after a single dose of WHO-approved vaccine. ## The Comprehensive Global Public Health Response The WHO, in close coordination with national governments and partners, is actively bolstering capacities for yellow fever preparedness and response worldwide. This involves a multi-pronged approach: **Enhanced Surveillance and Early Warning:** Robust surveillance systems are crucial for early detection. This includes systematic investigation and laboratory testing of all suspected human cases, active case finding in affected and neighboring areas, and retrospective review of death records. A key strategy in the Americas involves *epizootic surveillance* – monitoring yellow fever illness and deaths in non-human primates. Since monkeys develop symptoms before humans, their illness serves as an early warning sign, allowing health authorities to implement rapid response measures like targeted human vaccination and vector control. **Strengthening Laboratory Diagnostics:** Accurate and timely diagnosis is paramount. WHO supports countries in establishing and expanding laboratory capabilities for molecular testing (RT-PCR) to detect the virus in early stages of illness and for post-mortem diagnosis using tissue samples. While serological tests detect antibodies, their interpretation can be complex due to cross-reactivity with other similar viruses and recent vaccination, highlighting the need for a comprehensive approach combining lab results with epidemiological context. **Improved Clinical Management:** For patients who develop severe yellow fever, there is no specific antiviral treatment. Clinical management focuses on early detection, close monitoring, and supportive care to manage symptoms and complications like liver failure. WHO provides technical guidance and training for healthcare workers to enhance their ability to assess severity, refer patients, and manage severe cases effectively. **Vector Control and Risk Communication:** Alongside vaccination, controlling the mosquito population is vital. This involves entomological investigations to understand transmission risks and implementing measures to reduce mosquito breeding sites and protect communities from bites. Equally important is effective *risk communication* and community engagement. This involves educating communities, health workers, and travelers about the disease, promoting vaccination, encouraging reporting of animal disease outbreaks, and addressing misinformation. Clear, targeted messages are essential to foster public trust and adherence to preventive measures. ## WHO's Risk Assessment and Advice As of June 17, 2026, the WHO assesses the risk of yellow fever transmission as **low globally** but **moderate in regions with historical transmission**, specifically the WHO African Region and the WHO Region of the Americas. Unvaccinated individuals in rural or forest-edge communities, as well as those in urban areas with low immunity and competent mosquito vectors, face the highest risk. Travelers to high-transmission areas who are not vaccinated are also vulnerable. While vaccination offers strong protection, low coverage, fragile health systems, and delayed diagnoses in some areas continue to fuel the risk of outbreaks. WHO strongly advises Member States to maintain continuous surveillance and vaccination efforts in at-risk areas. It is crucial to achieve and sustain high vaccination coverage (over 80%) in at-risk populations and ensure a strategic reserve of vaccines for routine programs and outbreak responses. While yellow fever can be severe, WHO does not recommend any restrictions on international travel or trade to or from the affected countries based on the current situation. Instead, continuous traveler education on preventive measures, particularly vaccination, is strongly encouraged to ensure global health security. Maintaining vigilance, strengthening health systems, and ensuring widespread vaccination remain the most effective strategies to protect populations and prevent severe yellow fever epidemics worldwide.