Yellow fever vaccine
The vaccine consists of a live, but attenuated, strain of the yellow fever virus called 17D. The 17D vaccine has been used commercially since the 1950s. The mechanisms of attenuation and immunogenicity for the 17D strain are not known. However, this vaccine is very safe, with few adverse reactions having been reported and millions of doses administered, and highly effective with over 90% of vaccinees developing a measurable immune response after the first dose.
In 2013, the World Health Organization concluded, "a single dose of vaccination is sufficient to confer life-long immunity against yellow fever disease."
In 1937, Max Theiler, working at the Rockefeller Foundation, developed a safe and highly efficacious vaccine for yellow fever that gives a ten-year or more immunity from the virus. For his work on the yellow fever vaccine, he received the 1951 Nobel Prize in Physiology or Medicine.
The yellow fever 17D vaccine is considered very safe with over 500 million doses given and very few documented cases of vaccine associated illness. In no case of vaccine related illness has there been evidence of the virus reverting to a virulent phenotype.
The majority of adverse reactions to the 17D vaccine result from allergic reaction to the eggs in which the vaccine is grown. Persons with known egg allergy should discuss this with their physician prior to vaccination. In addition, there is a small risk of neurologic disease and encephalitis, particularly in individuals with compromised immune systems and very young children. The 17D vaccine is contraindicated in (among others) infants between 0-6 months of age, people with Thymus disorders associated with abnormal immune cell function, people with primary immunodeficiencies, and anyone with a diminished immune capacity including those taking immunosuppressant drugs.
According to the travel clinic at the University of Utah Hospital, the vaccine presents an increased risk of adverse reaction in adults aged 60 and older, with the risk increasing again after age 65, and again after age 70. The reaction is capable of producing multiple organ failure and should be evaluated carefully by a qualified health professional before being administered to the elderly.
There is a small risk of more severe yellow fever-like disease associated with the vaccine. This reaction, called YEL-AVD, causes a fairly severe disease closely resembling yellow fever caused by virulent strains of the virus. The risk factor/s for YEL-AVD are not known, although it has been suggested that it may be genetic. The 2`-5` oligoadenylate synthetase (OAS) component of the innate immune response has been shown to be particularly important in protection from Flavivirus infection. In at least one case of YEL-AVD, the patient was found to have an allelic mutation in a single nucleotide polymorphism (SNP) of the OAS gene.
The Canadian Medical Association published a 2001 CMAJ article entitled "Yellow fever vaccination: be sure the patient needs it". The article begins by stating that of the 7 people who developed system failure within 2 to 5 days of the vaccine in 1996–2001, 6 died "including 2 who were vaccinated even though they were planning to travel to countries where yellow fever has never been reported." The article cites that "3 demonstrated histopatholic changes consistent with wild yellow fever virus." The author recommends vaccination for only non-contraindicated travelers (see the articles list) and those travelers going where yellow fever activity is reported or in the endemic zone which can be found mapped at the CDC website cited below. In addition, the 2010 online edition of the Center for Disease Control Traveler's Health Yellow Book that between 1970 and 2002 only "nine cases of yellow fever were reported in unvaccinated travelers from the United States and Europe who traveled" to West Africa and South America, and 8 of the 9 died. However, it goes on to cite "only 1 documented case of yellow fever in a vaccinated traveler. This nonfatal case occurred in a traveler from Spain who visited several West African countries in 1988".
Candidates for vaccination
People most at risk of contracting the virus should be vaccinated. Woodcutters working in tropical areas should be particularly targeted for vaccination. Insecticides, protective clothing, and screening of houses are helpful, but not always sufficient for mosquito control; people should always use an insecticide spray while in certain areas. In affected areas, mosquito control methods have proven effective in decreasing the number of cases.
Travelers should have the vaccine 10 days prior to being in an endemic area. A booster shot may be required every 10 years if living in areas with yellow fever.