Yellow Fever Vaccine Information

Yellow Fever Vaccine Information

Learn more about the Yellow Fever Vaccine information. The form used for international travel thatcomplies with the IHR (2005) guidelines is the International Certification of Vaccination or Prophylaxis (CDC form 731, fomerly PHS 731)

The following information is taken from

Mark D. Gershman, J. Erin Staples

Yellow fever virus (YFV) is a single-stranded RNA virus that belongs to the genus Flavivirus.

Vectorborne transmission occurs via the bite of an infected mosquito, primarily Aedes or Haemagogus spp. Nonhuman and human primates are the main reservoirs of the virus, with anthroponotic (human-to-vector-to-human) transmission occurring. There are 3 transmission cycles for yellow fever: sylvatic (jungle), intermediate (savannah), and urban.

The sylvatic (jungle) cycle involves transmission of the virus between nonhuman primates and mosquito species found in the forest canopy. The virus is transmitted via mosquitoes from monkeys to humans when the humans encroach into the jungle during occupational or recreational activities.
In Africa, an intermediate (savannah) cycle involves transmission of YFV from tree hole-breeding Aedes spp. to humans living or working in jungle border areas. In this cycle, the virus may be transmitted from monkeys to humans or from human to human via these mosquitoes.
The urban cycle involves transmission of the virus between humans and peridomestic mosquitoes, primarily Ae. aegypti.

Humans infected with YFV experience the highest levels of viremia and can transmit the virus to mosquitoes shortly before onset of fever and for the first 3–5 days of illness. Given the high level of viremia, bloodborne transmission theoretically can occur via transfusion or needlesticks. One case of perinatal transmission of wild-type YFV has been documented from a woman who developed symptoms of yellow fever 3 days before giving birth. The infant tested positive for YFV RNA and died of fulminant yellow fever on the 12th day of life.

Yellow fever occurs in sub-Saharan Africa and tropical South America, where it is endemic and intermittently epidemic (see Tables 3-22 and 3-23 for a list of countries with risk of YFV transmission). Most yellow fever disease in humans is due to sylvatic or intermediate transmission cycles. However, urban yellow fever occurs periodically in Africa and sporadically in the Americas. In areas of Africa with persistent circulation of YFV, 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.

A traveler’s risk for 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 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 low level of transmission, a high level of immunity in the population (because of vaccination, for example), or failure of local surveillance systems to detect cases. This “epidemiologic silence” does not equate to absence of risk and should not lead to travel without taking protective measures.

YFV 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, YFV may be episodically transmitted by Ae. 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 Ae. aegypti in many towns and cities, South America is at risk for a large-scale urban epidemic.

From 1970 through 2015, a total of 10 cases of yellow fever were reported in unvaccinated travelers from the United States and Europe who traveled to West Africa (5 cases) or South America (5 cases). Eight (80%) of these 10 travelers died. There has been only 1 documented case of yellow fever in a vaccinated traveler. This nonfatal case occurred in a traveler from Spain who visited several West African countries in 1988. In early 2016, >15 long-term travelers from Africa and Asia developed yellow fever disease after visiting Angola, where one of the largest urban outbreaks was occurring. Reportedly, none of the ill travelers was vaccinated.

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 estimated 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

The risk of illness during outbreaks of the disease is likely higher. These estimates are a rough guideline based on the risk to indigenous populations, often during peak transmission season. Thus, these risk estimates may not accurately reflect the risk to travelers, who may have a different immunity profile, take precautions against getting bitten by mosquitoes, and have less outdoor exposure.

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

Burkina Faso
Central African Republic
Congo, Republic of the
Côte d’Ivoire
Democratic Republic of the Congo2
Equatorial Guinea Ethiopia2
Gambia, The
Mali2 Nigeria
Sierra Leone
South Sudan
Uganda Argentina2
French Guiana
Trinidad and Tobago2
Venezuela 2

Countries or areas where “a risk of YFV transmission is present,” as defined by the World Health Organization, are countries or areas where “yellow fever has been reported currently or in the past, plus vectors and animal reservoirs currently exist” (see the current country list within the International Travel and Health publication (Annex 1) at

2 These countries are not holoendemic (only a portion of the country has risk of yellow fever transmission). See Maps 3-14 and 3-15 and yellow fever vaccine recommendations (Yellow Fever & Malaria Information, by Country) for details.

Table 3-23. Countries with low potential for exposure to yellow fever virus (YFV)1

São Tomé and Príncipe3

1 Countries listed in this table are not contained on the official World Health Organization list of countries with risk of YFV transmission (Table 3-22). Therefore, proof of yellow fever vaccination should not be required if traveling from any of these countries to another country with a vaccination entry requirement (unless that country requires proof of yellow fever vaccination from all arriving travelers; see Table 3-26). An exception is Bolivia, which requires yellow fever vaccination for people traveling from or transiting through any of the 6 countries with low potential for exposure, in addition to those with risk of YFV transmission.

2 These countries are classified as “low potential for exposure to YFV” in only some areas; the remaining areas of these countries are classified as having no risk of exposure to YFV.
3The entire area of these countries is classified as “low potential for exposure to YFV.”

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