Yellow Fever and the Yellow Fever Vaccine

While yellow fever may be rare for visitors to northern countries, including the United States, it is transmitted by infected mosquitoes, so care should always be taken. For most people, there will be little to no symptoms of the illness. Few will go on to have severe problems, potentially including multi-organ failure. Treatment is based solely on symptoms, meaning there is no cure, but only palliative and symptomatic treatments. Therefore, prevention is particularly important.

What is yellow fever?

Yellow fever is caused by a virus and is also known as the yellow plague or yellow jack. It derives all of its names from the jaundice (due to liver damage) that occurs in advanced cases, which turns the skin and eyes yellow. The yellow fever virus is an RNA virus of the Flavivirus genus, which also includes West Nile virus and Dengue Fever. It is one of the viral hemorrhagic fevers, thus named because they may lead to fever and hemorrhaging. There are four types of yellow fever, two in Africa and two in South America, but only three of these have caused outbreaks. Up to half of the people who develop severe illness from yellow fever will die about 10 – 14 days later. The rest typically make a full or nearly full recovery.

Where does yellow fever come from?

Whereas the origins of the disease most likely lie in Africa, the first known outbreak occurred in Barbados in 1647. Yellow fever is typically found in tropical and sub-tropical areas, though mainly in South America and Africa. It was also found in the southern United States, though that was mostly true in times of war. In fact, yellow fever could have easily decided the Spanish-American War, as it devastated many troops on both sides. Less than one-quarter of Spanish troops in Cuba were well enough to fight by the time the Americans arrived. Walter Reed first discovered that it was transmitted via the bite of a mosquito while studying yellow fever just outside of Havana at the end of the conflict, which was around the turn of the 20th century. Max Theiler developed the first vaccine for the disease in 1937.

How is yellow fever transmitted?

Yellow fever is transmitted to humans through mosquitoes. The so-called “yellow fever mosquito” Aedes aegypti is the main vector for yellow fever. However, the disease has spread from several other species of mosquito as well, including the tiger mosquito. The virus enters female mosquitoes when they feed on infected primates. The virus may then replicate inside of the insect, eventually reaching the bloodstream and then the saliva. Female mosquitoes inject the infected saliva into future victims and thus introduce the virus into their bloodstreams. In some cases, the virus will transfer from a mother mosquito to her eggs. Yellow fever virus has three transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban.

What are yellow fever symptoms?

Yellow fever begins after an incubation period of 3 – 6 days. The symptoms of yellow fever are quite varied. Most victims will have very mild symptoms or none at all. Others will present with fever, pain, headache, nausea, vomiting and fatigue. About 85% of cases resolve without any further progression of the disease after 3 – 4 days. However, 15% of cases will go into remission for a few hours or a few days. Then, the disease will progress even further and cause symbols including higher fever, jaundice, bleeding in the eyes and mouth, seizures, coma and even organ failure. Severely ill people also often vomit blood due to bleeding in the gastrointestinal tract, hence the Spanish name for yellow fever, vomito negro (“black vomit). Up to half of the people who progress to the so-called “toxic phase” of yellow fever will die. Surviving yellow fever infections provides lifelong immunity.

What is the yellow fever vaccine?

The yellow fever vaccine is a single-dose live, attenuated vaccine that is given via subcutaneous (or intramuscular) injection. It contains the 17D strain of yellow fever and is therefore named the 17D vaccine. Credit is given to Max Theiler for developing a vaccine, which became commercially available in the 1950s. The vaccine is considered very safe and offers 90% of people immediate, measurable immunity. Boosters for the yellow fever vaccination are suggested every decade, but the World Health Organization states that a single dose is enough for lifetime immunity. The vaccine is recommended for individuals nine months of age or older who are living in or traveling to a country where yellow fever is common or where yellow fever vaccinations are compulsory.

What are the yellow fever vaccine side effects?

Most people who experience side effects from the yellow fever vaccine will experience mild symptoms, such as low fever, muscle aches and headaches. The majority of adverse reaction stems from an allergic reaction to the eggs in which the vaccine is grown. Persons with allergies may experience anaphylaxis or other, milder allergic reactions. Yellow fever related neurological disease, yellow fever related viscerotropic disease, as well as encephalitis are also possible side effects. All severe reactions are very rare.

Who should not get the yellow fever vaccine?

There is generally no reason to get the yellow fever vaccine if a person does not live in an afflicted area and/or does not plan to travel to one. The vaccine should never be given to infants six months old and younger, people with primary immunodeficiencies, symptomatic HIV infection sufferers, those undergoing immunosuppressive and/or immunomodulatory treatments, transplant recipients, those with malignant neoplasms. Extra caution should be taken when vaccinating the elderly, asymptomatic HIV patients, infants six to eight months old and pregnant/breastfeeding mothers.

Sources:

http://www.cdc.gov/yellowfever/vaccine/index.html

http://www.tmb.ie/destinations/vaccine.asp?disease=Yellow+Fever

http://www.who.int/mediacentre/factsheets/fs100/en

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