The year 2017 has barely begun and we already have a new and potentially major infectious disease situation flaring up. During the first three weeks of 2017, the dreaded deadly Yellow Fever virus (YFV) has erupted into an outbreak in Brazil. In many ways the past year has been a record-setting year for the heightened activity and geographic spread of Yellow Fever (YF) in the last several decades, with outbreaks in two very different parts of the world. These two distant locations, one in western sub-Saharan Africa and the other across the Atlantic Ocean in South America, have been threatening hideouts for the virus for several decades.


So far, the Brazilian outbreak is new and still somewhat limited, but up to 300 people may have already been infected, making this one of South America’s largest outbreaks of YF in recent years. As of 23 January 2017, the Ministry of Health has confirmed 25 human fatalities due to YF in Minas Gerais state in 2017, along with 47 confirmed and 206 suspected and probable cases. This is far more than all the cases combined for 2015 and 2016, and the governor has declared a state of emergency in the eastern part of the state. Additional points of concern have been a high number of cases of “jungle YF” in rural areas of Brazil, Peru, and Colombia over the past year, and the confirmation of a monkey death from YF near the urban area of Rio de Janeiro in late 2016. Movement of the virus into densely populated areas with breeding sites for vector mosquitoes near human habitations with inadequate refuge from mosquitoes, resulting in “urban YF”, is the biggest threat. Vaccination for YF is routine in many parts of Brazil, but a new vaccination campaign has been launched due to the currently unfolding situation.

Beginning in late 2015 and early 2016, western Africa experienced its worst YF outbreak in several decades, starting in Angola and expanding to the Democratic Republic of the Congo, with movement of YF cases to China through workers returning from Angola. This resulted in the first cases of YF ever reported from China, though fortunately it did not spread locally. The 2016 Africa outbreak was severe, taking several months of an aggressive UN-backed vaccination campaign and other measures before it was finally declared over just a month ago, on 23 December 2016. Limitations in stockpiling, production, and distribution of vaccine were considered to be major concerns in that outbreak. Difficulties in enforcing proof of vaccination among transborder migrants also was a problem, including cases of counterfeited vaccination cards. In a potential future scenario, simultaneous outbreaks in both Africa and South America could result in insufficient vaccine supplies with catastrophic results.

The virus is highly infectious to humans, to whom it is transmitted primarily by Aedes aegypti mosquitoes that prefer living near human homes, and are abundant throughout much of the world’s tropical, subtropical, and south temperate climates. Eradicating the disease has proven unlikely due to the fact that YF also infects wild primates, where it is transmitted endemically by other species of jungle-dwelling mosquitoes. Thus, the disease has persisted over the decades in a jungle cycle, from which it constantly threatens to erupt in a potentially devastating urban cycle when people visiting rural areas become infected and return to the city.

Since 2010, I have been working to raise awareness about the dangers of Yellow Fever re-emerging across a large geographic area of the globe. The danger is apparent from the fact that YF was once well established as far north as the United States, where it caused thousands of human deaths up through the late 1800s, before being driven out by aggressive mosquito control and the development of an effective vaccine. But the Aedes and other mosquito populations are spreading globally, and are back to dangerous levels in many locations, as borne out in the completely unprecedented emergence of Chikungunya virus and Zika virus in the Americas in recent years, along with continued spread of Dengue virus globally. These circumstances, along with the YF situations on two continents in the past year, have underscored the need for immediate and sustained action, as well as revealing the weak points in our preparedness for a potential Yellow Fever pandemic.

Prof. Dr. David Bruce Conn is a biomedical scientist who has conducted research, taught university students, and advised governments and industries about parasitic and other infectious diseases, invasive species, and environmental health for 40 years. (Disclaimer: Dr. Conn is a senior advisor to the U.S. Department of State on issues of health and foreign policy, but the contents of this post are his own views and do not officially represent any unit of the U.S. federal government).