Epidemiology
Yellow fever is endemic in sub-Saharan Africa, Central and South America, and the Caribbean, with most cases (approximately 90%) occurring in Africa. Globally, an estimated 84,000 to 170,000 severe cases occur annually, resulting in 29,000 to 60,000 deaths.[3] Factors such as the lack of diagnostic facilities in, and remoteness of, outbreak areas, result in under-reporting that impedes the evaluation of the true disease burden.
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The most recent outbreak reported was in Mali. As of 26 December 2019, 3 confirmed cases (with 2 deaths), 9 suspected cases, and 3 probable cases have been reported.[4]
Outbreaks have been reported in Nigeria since 2017.[5] There was nearly a three-fold increase in the number of confirmed cases in 2019 compared with 2018, suggesting intensification of transmission. From 01 January to 10 December 2019, a total of 4189 suspected cases were reported, with 115 confirmed cases and 23 deaths.[6]
A large outbreak occurred in Angola and the Democratic Republic of the Congo between December 2015 and July 2016, with more than 7300 suspected and 900 confirmed cases.[7] Outbreaks were also reported in China (11 cases were imported from Angola), Uganda, and Kenya in 2016.
An outbreak in Brazil was first reported in December 2016 and declared over in September 2017. However, there has been a resurgence of human cases since December 2017, along with a rise in nonhuman primate epizootics since September 2017. As of June 2018, at least 1266 confirmed human cases were reported with 415 deaths. Cases have been reported in São Paulo, Minas Gerais, Espirito Santo, Rio de Janeiro, and in the Federal District. Several cases have been confirmed in unvaccinated travelers returning from Brazil, with a case reported in a returning traveler in the UK.[8]
Yellow fever has the potential for rapid spread by international travelers to vulnerable countries whereAedes aegypti mosquitoes are present. An example of this occurred in 2016 when 11 infected travelers entered China from Angola, thereby putting a largely unvaccinated population at risk. Fortunately, autochthonous transmission did not occur.[9]
In the US, all cases are imported and occur in unimmunized travelers to areas of risk who have not taken appropriate mosquito precautions. A total of 11 cases were reported in unvaccinated travelers to West Africa and South America from the US and Europe between 1970 and 2015; 8 of those people died. Only one traveler had a documented history of vaccination, and that person survived. Outbreaks in Angola and Brazil between 2016 and mid-2021 resulted in more than 37 travel-associated cases reported in unvaccinated travelers who were residents of nonendemic areas or countries.[10]
Although all American countries with enzootic areas have incorporated yellow fever vaccination into their routine childhood immunization programs, 22 confirmed cases were reported from the region of the Americas (Bolivia, Brazil, and Peru) during 2010. The case-fatality rate was very high, at 77.3% (17 deaths). No cases were found in urban environments.[11] Although this was the lowest number of confirmed cases since 1967, nearly all major urban centers in the American tropics have been reinfested with Aedes aegypti, and most urban dwellers and travelers are vulnerable because of low immunization coverage. The WHO stated in 2017 that "Latin America is now at greater risk of urban epidemics than at any time in the past 50 years."[12]
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