Yellow fever risk factors

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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

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.

Risk Factors

• Transmission in rural West Africa is seasonal, with an elevated risk during the end of the rainy season and the beginning of the dry season (usually July–October). *However, yellow fever virus may be episodically transmitted by Aedes aegypti even during the dry season in both rural and densely settled urban areas.
• The risk for infection in South America is highest during the rainy season (January–May, with a peak incidence in February and March).
• Given the high level of viremia that may occur in infected humans and the widespread distribution of Aedes aegypti in many towns and cities, South America is at risk for a large-scale urban epidemic.
• 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.The risk of acquiring yellow fever is difficult to predict because of variations in ecologic determinants of virus transmission.
• 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.
• The risk of acquiring yellow fever in South America is lower than that in Africa, because the mosquitoes that transmit the virus between monkeys in the forest canopy in South America do not often come in contact with humans.
• Additionally, there is a relatively high level of immunity in local residents because of vaccine use, which might reduce the risk of transmission.

Countries with risk of yellow fever virus (YFV) transmission

Africa			Central and South America
Angola Benin Burkina Faso Burundi Cameroon Central African Republic Chad† Congo Côte d’Ivoire Democratic Republic of the Congo†	Equatorial Guinea Ethiopia† Gabon Gambia Ghana Guinea Guinea-Bissau Kenya† Liberia Mali†	Mauritania† Niger† Nigeria Rwanda Senegal Sierra Leone Sudan† South Sudan Togo Uganda	Argentina† Bolivia† Brazil† Colombia† Ecuador† French Guiana Guyana Panama† Paraguay Peru† Suriname Trinidad and Tobago† Venezuela†
†These countries are not holoendemic (only a portion of the country has risk of yellow fever transmission). Table adapted from CDC