Yellow fever (as per WHO)

Cause

Yellow fever virus (YFV), an arbovirus of the Flavivirus genus.

Transmission

Yellow fever occurs in urban and rural areas of Africa and central South America. In jungle and forest areas, monkeys are the main reservoir of infection, which is spread by mosquitoes from monkeyto- monkey and, occasionally, to humans. In urban settings mosquitoes transmit the virus from human-to-human and introduction of infection into densely populated urban areas can lead to large epidemics of yellow fever. In Africa, an intermediate pattern of transmission is common in humid savannah regions where mosquitoes infect both monkeys and humans, causing localized outbreaks.

Nature of the disease

Although most infections are asymptomatic, some lead to an acute illness characterized by two phases. Initially, there is fever, muscular pain, headache, chills, anorexia, nausea and/or vomiting, often with bradycardia. About 15% of patients progress to a second phase after a few days, with resurgence of fever, development of jaundice, abdominal pain, vomiting and haemorrhagic manifestations; up to half of these patients die 10–14 days after the onset of illness.

Geographical distribution

In tropical areas of Africa and Central and South America (see maps) YFV transmission can occur at altitudes up to 2300 metres (in Africa, possibly higher). Countries or areas where the YFV is present far exceed those officially reported. Some countries may have no reported cases simply because of a high level of vaccine coverage against yellow fever, or because of poor surveillance. A revision of the risk classification of countries and areas recommended for yellow fever vaccination is reflected in this year’s edition of IT&H (Country List and Annex 1).

Risk for travellers

Apart from areas of high yellow fever endemicity, YFV transmission may take place also in low-endemic areas if the traveller’s itinerary implies heavy exposure to mosquitoes, for example during prolonged travel in rural areas.

General precautions

Avoid mosquito bites; the highest risk for YFV transmission is during the day and early evening

Vaccine

The 17D vaccine, which is based on a live, attenuated viral strain, is the only commercially available yellow fever vaccine. It is given as a single subcutaneous (or intramuscular) injection. Yellow fever vaccine is highly effective (approaching 100%). All individuals aged 9 months or older and living in countries or areas at risk should receive yellow fever vaccine.