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Yellow Fever: A Vaccine-Preventable Infectious Disease Emergency

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, July 8, 2016

In the modern era, when one speaks of infectious diseases spread by Aedes mosquitos, dengue and chikungunya are the first to come to mind, as they are currently very clinically relevant. That changed with the appearance and explosive spread of Zika in the Western Hemisphere. However, even more recently the world has been reminded, in a major way, of the original infectious disease found to be spread by an Aedes mosquito: yellow fever.

Since late 2015, a yellow fever outbreak has been ongoing in the African nation of Angola, with thousands of cases accruing and travelers to several other nations, including China, being diagnosed. Like other mosquito disease–infected travelers in areas with the requisite mosquito present, they can seed local mosquito populations and spark local outbreaks. The seriousness of the Angolan outbreak has prompted the World Health Organization to convene its emergency committee.


Majority of Yellow Fever Cases Asymptomatic

Yellow fever is a flavivirus, and, like many members of this family, the majority of infections with this virus (85%), spread via the Aedes aegypti mosquito, are asymptomatic or mild. If a person is severely symptomatic, however, infection can culminate in fatal hemorrhagic fever. There is no specific antiviral treatment for the virus.

Yellow fever is endemic in many nations in Africa and South America, and these cases lead to nearly 80,000 deaths annually. In areas of endemicity, the yellow fever vaccine is the chief means of prevention. Vector control activities are also crucial.


Vaccine Shortage

The control of this outbreak has been plagued by vaccine shortages. Mass vaccination with the yellow fever vaccine is being used as a major means to extinguish the outbreak; however, the yellow fever vaccine stockpile does not have enough doses to cover the population adequately, leading to the recommendation to dilute the vaccine to stretch supply and later provide a 2nd booster dose (which isn’t needed if the full dose is administered). 


A Vaccine-Preventable Outbreak

This yellow fever outbreak is notable for being entirely vaccine preventable. The fact that it occurred underscores the limitations of the world’s ability to rapidly (and effectively) respond to an infectious disease emergency. An outbreak of this magnitude and with this degree of morbidity and mortality should not have been possible given that an effective vaccine has been available for several decades, its manufacture is well established, and it is stockpiled. 



European Centre for Disease Prevention and Control. Mission Report: Assessing the Yellow Fever Outbreak in Angola. European Medical Corps mission undertaken in the framework of the European Union Civil Protection Mechanism, 10–20 May 2016. Stockholm: ECDC; 2016. Accessed July 6, 2016.