Yellow Fever

Yellow fever is a viral disease that is transmitted to humans through the bite of infected mosquitoes. Illness ranges in severity from an influenza-like syndrome to severe hepatitis and hemorrhagic fever. The yellow fever virus is maintained in nature by mosquito-borne transmission between nonhuman primates. Transmission by mosquitoes from one human to another occurs during epidemics of "urban yellow fever."

Risk for Travelers

A traveler's risk of acquiring yellow fever is determined by immunization status, location of travel, season, duration of exposure, occupational and recreational activities while traveling, and the local rate of yellow fever virus transmission at the time. Although reported cases of human disease are the principal indicator of disease risk, they may be absent (because of a high level of immunity in the population) or not detected as a result of poor surveillance. Only a small proportion of yellow fever cases are officially reported because of the occurrence of the disease in remote areas and lack of specific diagnostic facilities.

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" may provide a sense of false security and lead to travel without the benefit of vaccination. Surveys in rural West Africa during "silent" periods have estimated an incidence of yellow fever of 1.1-2.4 cases per 1,000 persons and an incidence of death due to yellow fever of 0.2-0.5 deaths per 1,000 persons; both these ranges are less than the threshold of detection of the surveillance systems in place.

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; these risks vary greatly according to specific location and season. In West Africa, the most dangerous time of year is during the late rainy and early dry seasons (July-October). Virus transmission is highest during the rainy season (January-March) in Brazil.