The yellow fever virus, an arbovirus of the Flavivirus genus.
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 monkey-to-monkey and, occasionally, to humans. These mosquitoes bite during daylight hours. 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.
Tropical areas of Africa and South America (Map). Transmission can occur at altitudes up to 2300 m in the Americas and possibly higher in Africa. Countries or areas where the yellow fever virus 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 in the population 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 (Country List and Annex 1).
Risk for travellers
Travellers are at risk in all countries or areas at risk of yellow fever transmission (Country List and Annex 1). Yellow fever vaccination is generally not recommended in areas where there is low potential for exposure to yellow fever virus. However, vaccination might be considered for a small subset of travellers to these areas, who are at increased risk of exposure to yellow fever virus (e.g. prolonged travel, extensive exposure to mosquitoes, inability to avoid mosquito bites). When considering vaccination, any traveller must take into account the risk of being infected with yellow fever virus, country entry requirements, as well as individual risk factors (e.g. age, immune status) for serious vaccine-associated adverse events.
Avoid mosquito bites during the day and early evening (Chapter 3).