Yellow fever virus in endemic (constantly present in low numbers) in 33 different countries in Africa (mostly West and Central Africa, especially during the rainy season), a number of South American countries (especially Bolivia, Brazil, Colombia, Ecuador and Peru), and in several of the Caribbean islands (especially Trinidad and Tobago). Map of geographic yellow fever distribution
Although most cases of yellow fever in South America are associated with sylvatic transmission (Yellow fever) due to agricultural and logging activities in the rainforests, there are also cases of urban transmission that occur. In Africa, transmission may be related to sylvatic, intermediate or urban transmission cycles.
Prevention of yellow fever includes immunization prior to travel to endemic areas, and general precautions to avoid mosquito bites. Although yellow fever is a rare cause of illness in travelers, some countries have regulations for yellow fever vaccination that must be met before entering the country, especially when prior travel has involved countries where yellow fever is endemic.
Yellow fever vaccine has been available for several decades, and a single dose provides immunity for 10 or more years. Yellow fever vaccine is a live attenuated (weakened) virus preparation made from the 17D yellow fever virus strain. The 17D strain of yellow fever was developed by serially infecting mice until the virus was no longer virulent (Max Theiler Nobel Lecture).
Yellow fever vaccine can be safely administered to adults and children over 9 months of age. The yellow fever vaccine is reportedly one of the safest vaccines available, with fewer than 5% of those immunized developing side-effects such as mild headache, muscle pain, fatigue and other minor symptoms 5-10 days following immunization. However, because the vaccine is propagated in chicken embryos, its use in persons with
Severe side-effects following yellow fever immunization are rare, but do occur (less than 1/400,000 persons vaccinated). Severe side-effects include hypersensitivity, neurotropic disease and viscerotropic disease:
Hypersensitivity
Because yellow fever vaccine is produced in chick embryos, it should not be administered to persons with egg allergies. If vaccination of a person with egg hypersensitivity is required, an intradermal test dose can be administered under close medical supervision.
Yellow Fever Vaccine-Associated Neurotropic Disease
- previously - febrile multiple organ system failure,
- now - yellow fever vaccine-associated viscerotropic disease
- may result in severe multiple organ system failure
- only 10 cases have been reported (6 of the 10 died)
Yellow Fever Vaccine-Associated Viscerotropic Disease
- Symptoms include fever, hypotension, respiratory failure, elevated hepatocellular enzymes, hyperbilirubinemia, lymphocytopenia, and thrombocytopenia
- Only 4 cases have been reported (3 of the 4 died)
Yellow fever vaccine can only be obtained at designated yellow fever vaccination centers. The need for immunization is best decided following a consultation with a Travel Medicine Clinic professional:
Once immunized for yellow fever, you will receive an International Certificate of Vaccination that serves as official documentation of immunization if required by a country for entry. A waiver letter must be obtained prior to travel to a country with yellow fever entry regulation if conditions exist that contraindicate yellow fever immunization.
Avoiding mosquito bites:
- Wear clothes that cover the arms and legs.
- Wear light-colored clothing (mosquitos are attracted to dark colors).
- Do not wear scented products.
- Apply DEET-containing insect repellent to skin and clothing every 6 hours.
- Remain in well-screened or air-conditioned areas when possible.
- Use mosquito netting.
Yellow fever vaccination:
Yellow fever vaccine generally has few side effects; fewer than 5% of vaccinees develop mild headache, muscle pain, or other minor symptoms 5 to 10 days after vaccination. Under almost all circumstances, there are four groups of people who should not receive the vaccine unless the risk of yellow fever disease exceeds the small risk associated with the vaccine. These people should obtain either a waiver letter prior to travel or delay travel to an area with active yellow fever transmission.
Yellow fever vaccine is contraindicated in the following groups of people:
- Infants under 6 months of age due to risk of viral encephalitis
- Pregnant women due to theoretical risk that vaccine strain may infect developing fetus
- Persons hypersensitive to eggs
Immunocompromised individuals where immune system has been altered by diseases (e.g. leukemia, lymphoma), infections (e.g. AIDS, HIV), drugs or radiation treatments.
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