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| YELLOW FEVER |
Disease
Yellow fever is the virus infection which is transmitted by AEDES and Jungle mosquitoes. The mosquitoes transmit the infection by first biting an individual having the disease and then biting a susceptible individual after the virus has multiplied within the mosquito’s body. The incubation period in humans is 3-6 days. Adults and children are equally susceptible, though attack rates are highest among adult males because of their work habits. Between 5% and 50% of infection are asymptomatic.
Occurrence
The yellow fever occurs in urban as well as jungle cycle Africa and in a Jungle cycle in South America. Epidemics have extended far into the temperate zone during warm seasons.
Risk for travellers
The normally low risk to travellers increases with travel to jungle areas in endemic countries and in or near cities during urban outbreaks. Areas where yellow fever virus is present far exceed those officially reported. The risk of exposure to infection can be reduced by taking measures to prevent mosquito bites (see Chapter 3).
It should be noted that the mosquito vectors of yellow fever bite mostly during daylight hours. Although reported cases of human disease are the principal indicator of disease risk, some countries may have no reported cases, either because of a high level of vaccine coverage against yellow fever in the population or because poor surveillance resulted in no cases being reported. However, the risk of yellow fever may still persist as the virus, the vector or the animal reservoir is still present.
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.
Live yellow fever vaccine is indicated for those traveling or living in areas where infection is endemic and for laboratory staff who handle the virus or who handle clinical material from suspected cases. Infants under 6 months of age should not be vaccinated because there is a small risk of encephalitis; infants aged 6-9 months should be vaccinated only if the risk of yellow fever is high and unavoidable. The immunity which probably lasts for life is officially accepted for 10 years starting from 10 days after primary immunization and for a further 10 years immediately after revaccination.
The vaccine should not be given to those with impaired immune responsiveness, or who have had an anaphylactic reaction to egg; it should not be given during pregnancy but if a significant risk of exposure cannot be avoided then vaccination should be delayed to the third trimester if possible (but the need for immunization usually outweighs risk to the fetus; however pregnant women should be advised not to travel to areas where exposure to yellow fever may occur). |
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