Historically, yellow fever (YF) has claimed millions of lives, including many thousands in the United States. The Philadelphia epidemic of 1793, for example, killed about 10 percent of the city’s population. While the last occurrence of yellow fever transmission in the U.S. was a major epidemic in New Orleans during 1905, the disease is still very active in tropical and subtropical areas of South America and Africa. Extensive immunization campaigns, along with effective mosquito control, especially in developed countries, have reduced yellow fever cases worldwide. However, localized outbreaks still occur in parts of Africa and Central and South America, accounting for an estimated 84,000 to 170,000 severe cases of disease and 29,000 to 60,000 deaths annually. From 1970 through 2015, a total of 10 cases of yellow fever were reported in unvaccinated travelers from the United States and Europe who traveled to West Africa (5 cases) or South America (5 cases). Eight of these 10 travelers died.
The virus is transmitted to people by the bite of an infected mosquito, primarily Ae. aegypti. As we have recently seen with Zika virus, arbovirus epidemics spread by Ae. aegypti can move rapidly through populations that lack immunity and can be readily spread by human travelers. The on-going yellow fever outbreak in Brazil is moving closer to major urban centers such as Rio de Janeiro and Sao Paolo, where tens of millions of people live. Health officials are worried that the virus could spread to city-dwelling Ae. aegypti mosquitoes and start the disease in an urban transmission cycle. Although it is unlikely that a yellow fever outbreak with local transmission will occur in the U.S., the potential does exist in warmer regions of the continental United States as well as in US territories in the Caribbean where Ae. aegypti is endemic.
According to CDC, yellow fever ranges in severity from a self-limited febrile illness to severe life-threatening disease. Symptoms include fever, chills, headache, backache, and muscle aches. About 15% of people who get yellow fever develop serious illness that can lead to bleeding, shock, organ failure, and sometimes death. There is no specific treatment for yellow fever and care is based on symptoms. Fortunately there is a very effective vaccine. In 2014, the WHO determined that a single immunization provides sustained immunity and lifelong protection against yellow fever disease.