Yellow fever is a vector-borne disease endemic in Africa and Latin America, causing an estimated 380,000 severe cases including up to 180,000 deaths annually in Africa where around 90% of the global burden occurs. Yellow fever is caused by a virus (genus Flavivirus) that can be transmitted to humans by the bites of infected aedes and haemogogus mosquitoes. The “yellow” in the name refers to the jaundice that affects some patients. Initial symptoms of yellow fever include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. A small proportion of patients develop severe symptoms and approximately half of those severe cases die within seven to ten days. There is no medical treatment for yellow fever, but a safe and long-lasting vaccine has been available since the 1930s. Complete elimination of yellow fever is not possible due to a large wildlife reservoir of the virus. Therefore, disease prevention through large-scale vaccination campaigns is the most effective approach for disease control.

During a yellow fever outbreak, vaccination remains the most effective measure for protecting the population at risk. In recent years, several large yellow fever outbreaks have caused considerable concern to the international community due to extreme vaccine shortages and the fear of export to as yet unaffected areas, most notably Asia. As of 2016, WHO has 6 million vaccines in an emergency stockpile managed by the International Coordinating Group (ICG). However, recent urban epidemics, such as the one in Luanda, Angola, have demonstrated that the population in need of the vaccine can significantly exceed the vaccine availability. Studies have therefore examined the efficacy of administering a fractional dose of the vaccine as a strategy to maximise the use of available resources. The initial results indicate that a fifth of the standard vaccine dose could provide sufficient protection from the virus, albeit with a large margin for error. More studies are urgently needed to understand the impact of such an approach in case the efficacy is lower than anticipated.

Model name: Yellow fever model (Imperial College)

WHO fact sheet on yellow fever
CDC page on yellow fever
Gavi page on yellow fever