These are areas where the virus is present in monkeys and is a potential risk to humans as defined by the World Health Organisation. Some of these countries demand a yellow fever certificate from travellers as a condition of entry to their country.

Yellow Fever Risk Areas

These are areas where the virus is present in monkeys and is a potential risk to humans as defined by the World Health Organisation. Some of these countries demand a yellow fever certificate from travellers as a condition of entry to their country. Many of these, and other countries, will ask you for a certificate if you are entering from an infected country. Your travel agent should inform you if you need a certificate when you are booking your holiday or flight. You can double check if in doubt with your local yellow fever vaccination centre.

Mode of transmission

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) transmission 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 transmission cycle involves transmission of the virus between humans and urban mosquitoes, primarily Aedes 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.

Epidemiology

Yellow fever occurs in sub-Saharan Africa and tropical South America, where it is endemic and intermittently epidemic (see Tables 1 and 2 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 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 or transitional areas.

Table 1. Countries with risk of yellow fever virus (YFV) transmission¹

¹Countries/areas where “a risk of yellow fever 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 www.who.int/ith/en/index.html).

²These countries are not holoendemic (only a portion of the country has risk of yellow fever transmission). See Maps 3-18 and 3-19 and yellow fever vaccine recommendations (Yellow Fever and Malaria Information, by Country) for details.

Table 2. Countries with low potential for exposure to yellow fever virus (YFV)¹

¹Countries listed in this table are not contained on the official WHO list of countries with risk of YFV transmission. Therefore, proof of yellow fever vaccination should not be required if traveling from one 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).

²These 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.”

Table 3. Countries that require proof of yellow fever vaccination from all arriving travelers