Yellow fever epidemiology and demographics

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]

Overview

Yellow fever occurs in sub-Saharan Africa and tropical South America, where it is endemic and intermittently epidemic. Urban yellow fever occurs periodically in Africa and sporadically in the Americas. In Africa, natural immunity accumulates with age, and thus, infants and children are at highest risk for disease. In South America, yellow fever occurs most frequently in unimmunized young men who are exposed to mosquito vectors through their work in forested areas.

Epidemiology and Demographics

Areas with Risk of Yellow Fever Virus Transmission in Africa Image obtained from CDC

Areas with Risk of Yellow Fever Virus Transmission in South America Image obtained from CDC

• Yellow fever occurs only in Africa, South and Central America, and the Caribbean.
• Most outbreaks in South America are to people who work within the tropical rain forests and have direct contact with the organisms within the rainforest.

• The disease can remain locally unknown in humans for long periods of time and then suddenly break out in an epidemic fashion.
• In Central America and Trinidad, such epidemics have been due to a form of the disease (jungle yellow fever) that is kept alive in Red Howler monkey populations and transmitted by Haemagogus mosquito species which live only in the canopy of rain forests.
• The virus is passed to humans when the tall rainforest trees are cut down. Infected woodcutters can then pass on the disease to others via species of Aedes mosquitoes that typically live at low altitudes, thus triggering an epidemic.

Incidence

• During interepidemic periods, low-level transmission may not be detected by public health surveillance.
• Such interepidemic conditions may last years or even decades in certain countries or regions.
• This “epidemiologic silence” does not equate to absence of risk and should not lead to travel without the protection provided by vaccination.
• Surveys in rural West Africa during “silent” periods have estimated an annual incidence of yellow fever of 1.1-2.4 cases per 1,000 persons and 0.2-0.5 deaths per 1,000 persons.
• Yellow fever viral transmission in rural West Africa is seasonal, with elevated risk during the 2-4 months that the rainy season ends and the dry season begins (usually July-October); therefore, the annual incidence reflects incidence during a transmission season of 2-4 months.

• The incidence of yellow fever in South America is lower than that in Africa because the mosquitoes that transmit the virus between monkeys in the forest canopy do not often come in contact with humans and because immunity in the indigenous human population is high.
• Urban epidemic transmission has not occurred in South America for many years, although the risk of introduction of the virus into towns and cities is ever present.
• For travelers, the risks of illness and death due to yellow fever are probably 10 times greater in rural West Africa than in South America; the risk varies greatly according to specific location and season.
• In West Africa, virus transmission is highest during the late rainy and early dry seasons (July-October). In Brazil, the risk of infection is highest during the rainy season (January-March) .
• The low incidence of yellow fever in South America, generally a few hundred reported cases per year, could lead to complacency among travelers.
• However, it is important to note that four of the six cases of yellow fever reported among travelers from the United States and Europe in 1996-2002 acquired yellow fever in South America .
• All six cases were fatal and occurred among unvaccinated travelers. An increase in enzootic and epizootic yellow fever transmission in South America during the 1990s and the potential for epidemiologic change in the Americas remains a concern

• The risk of acquiring yellow fever is difficult to predict because of variations in ecologic determinants of virus transmission.
• Based on data for U.S. travelers during 1996-2004, the overall risk for serious illness and death due to yellow fever in travelers has been roughly estimated to be 0.05 -0.5 per 100,000 travelers to yellow fever-endemic areas.
• This range reflects an unvaccinated population of 10-90% and assumes that all travelers visiting holo-endemic countries are at risk and 10% of travelers to non holo-endemic countries are visiting risk areas.

Risk for Travelers

• The risk of acquiring yellow fever is difficult to predict because of variations in ecologic determinants of virus transmission.
• For a 2-week stay, the risks for illness and death due to yellow fever for an unvaccinated traveler visiting an endemic area in:

• West Africa are 50 per 100,000 and 10 per 100,000, respectively
• South America are 5 per 100,000 and 1 per 100,000, respectively

• These estimates are based on risk to indigenous populations and may not accurately reflect the true risk to travelers, who may have a different immunity profile, take precautions against getting bitten by mosquitoes, and have less outdoor exposure.

References

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