Yellow fever vaccine

The yellow fever vaccine is a medical tool used to prevent a viral infection called yellow fever. This disease occurs primarily in tropical regions of Africa and South America.

Because the virus can cause serious illness, the vaccine is included on the World Health Organization's List of Essential Medicines. It is used globally to control outbreaks and protect people living in or traveling to endemic areas. Most people develop immunity within ten days of receiving the shot. Within one month, they are 99% protected. This protection appears to be lifelong for most individuals.

The vaccine works by using an attenuated virus. An attenuated virus is a version of the virus that has been weakened in a laboratory. It is still alive, but it is not strong enough to cause the actual disease in a healthy person. Instead, it teaches the immune system to recognize and fight the virus. The current version, known as the 17D vaccine, is grown using chicken eggs to help the virus multiply. The vaccine is administered as an injection into a muscle or just under the skin.

While the vaccine is generally safe, it is not suitable for everyone. It is contraindicated, or medically advised against, for certain groups. This includes infants between zero and six months old. It should also not be given to people with primary immunodeficiencies or thymus disorders. These conditions affect how the body's immune cells function. People taking immunosuppressant drugs should also avoid the vaccine. However, it is considered safe for people with HIV infection if they do not have active symptoms. It is also considered safe for use during pregnancy for those at risk of exposure.

Most side effects are mild, such as a headache, muscle pains, or a fever. However, more serious reactions can occur in very rare cases. Severe allergic reactions happen in about eight per million doses. Serious neurological problems occur in about four per million doses. Organ failure occurs in about three per million doses. There are also two specific rare conditions: YEL-AVD and YEL-AND. YEL-AVD, or yellow fever vaccine-associated acute viscerotropic disease, causes a severe illness that looks like the actual disease. YEL-AND is a neurotropic reaction involving the nervous system. Many adverse reactions are actually caused by allergies to the eggs used to grow the virus.

The history of the vaccine is a long journey of trial and error. In the early 20th century, Hideyo Noguchi mistakenly believed the disease was caused by a bacterium. Later, scientists used human serum to create vaccines, but this was difficult to produce in large amounts. In 1937, Max Theiler, working with Hugh Smith and Eugen Haagen, discovered a breakthrough. They found a mutation in the virus that created a highly effective, weakened strain called 17D. This discovery earned Theiler the Nobel Prize in Physiology or Medicine in 1951.

Since the 1930s, only 12 known cases of yellow fever have occurred after 600 million doses were given.

Global supply and demand for the vaccine are managed by organizations like the WHO and Gavi. In the mid-2000s, the WHO Yellow Fever Initiative was launched to address rising cases in Africa and South America. As of 2013, there were four WHO-qualified manufacturers in Brazil, Senegal, Russia, and France. Demand has grown significantly, with projections reaching 62 million doses per year by 2014. However, in 2013, UNICEF reported that supplies were insufficient to meet this need. Manufacturers were producing about 35 million of the 64 million doses required annually.

Vaccination is a key part of a larger strategy to manage the disease. Medical experts recommend using insecticides, protective clothing, and house screening to control mosquitoes. However, these methods are not always enough. People in endemic areas are encouraged to use personal insecticide spray. For travelers, timing is critical. They should receive the vaccine at least ten days before entering an endemic area. This ensures the body has enough time to build full immunity. Some countries even require a vaccination certificate for entry to prevent the spread of the virus.