Yellow fever risk factors
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Risk Factors
A traveler’s risk of acquiring yellow fever is determined by various factors, including immunization status, location of travel, season, duration of exposure, occupational and recreational activities while traveling, and the local rate of virus transmission at the time of travel. Although reported cases of human disease are the principal indicator of disease risk, case reports may be absent because of a high level of immunity in the population (e.g., due to vaccination campaigns), or because cases are not detected by local surveillance systems (1). Only a small proportion of yellow fever cases is recognized and officially reported because the involved areas are often remote and lack specific diagnostic capabilities. 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. YFV 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) (2). 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 (3-8). 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 (see Chapter 5).
The risk of acquiring yellow fever is difficult to predict because of variations in ecologic determinants of virus transmission. As a rough guideline, the risks of illness and death due to yellow fever in an unvaccinated traveler in endemic areas in West Africa during the highest risk season from July to October have been estimated at 100 per 100,000 and 20 per 100,000 per month, respectively; for a 2-week stay, the estimated risks of illness and death were 50 per 100,000 and 10 per 100,000, respectively (2). The risks of illness and death in South America are probably 10 times lower (5 per 100,000 and 1 per 100,000, respectively for a 2-week trip) (2). 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. 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.