Emergencies preparedness, response

Strategies and activities

Mission statement

- To support countries in preparing and responding effectively to yellow fever outbreaks.
- To link outbreak response at national level with long-term efforts to control yellow fever at regional and global levels.

The yellow fever initiative

Yellow fever is an acute, haemorrhagic viral disease transmitted to people of all ages by infected mosquitoes.

WHO and its partners established a Yellow Fever International Coordinating Group to oversee management of an emergency vaccine stockpile of 6 million doses by year.

Yellow fever is caused by a virus (Flavivirus) which is transmitted to humans by the bites of infected aedes and haemogogus mosquitoes. The mosquitoes either breed around houses (domestic), in forests or jungles (wild), or in both habitats (semi-domestic).

Once contracted, the yellow fever virus incubates in the body for 3 to 6 days. Infection follows in one or two phases.

1. The first, "acute", phase causes symptoms including fever, muscle pain, prominent backache, headache, shivers, loss of appetite, and nausea or vomiting. After 3 to 4 days, most patients improve and their symptoms disappear.

2. However, 15% of patients enter a second, more toxic phase within 24 hours of the initial remission. Symptoms include high, rapidly developing jaundice, abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach, after which blood appears in the vomit and faeces. Kidney function also deteriorates.

Half of the patients who enter the toxic phase die within 10 to 14 days, the rest recover without significant organ damage.

Yellow fever is difficult to diagnose, especially during the early stages, as its symptoms can be confused with other diseases such as severe malaria, leptospirosis, viral hepatitis, dengue hemorrhagic fever, other hemorrhagic fevers, as well as poisoning. Yellow fever can be diagnosed by blood tests that detect yellow fever antibodies produced in response to the infection. Several other techniques can also be used to identify the virus in blood specimens or liver tissue collected after death. These tests require highly trained laboratory staff and specialized equipment and materials.

There are three types of transmission cycle:

1. Sylvatic (or jungle): In tropical rainforests, yellow fever occurs in monkeys that pass the virus to mosquitoes that feed on them. The infected mosquitoes bite humans entering the forest resulting in sporadic cases of yellow fever, usually in young men working in the forest (e.g. loggers).

2. Intermediate: In humid or semi-humid parts of Africa, semi-domestic mosquitoes (mosquitoes that breed in the wild and around households) infect both monkeys and humans. Increased contact between people and infected mosquitoes leads to increased transmission and small-scale epidemics can occur. An outbreak can become a more severe epidemic if the infection is carried into an area populated with both domestic mosquitoes and unvaccinated people.

3. Urban: Large epidemics occur when infected people introduce the virus into a densely populated area with high numbers of non-immune people and Aedes mosquitoes. Infected mosquitoes transmit the virus from person to person.

There is no specific treatment for yellow fever – only supportive care to treat dehydration, respiratory failure and fever. Associated bacterial infections can be treated with antibiotics. Supportive care may improve outcomes for seriously ill patients, but may not be available in resource-poor contexts.

Vaccination is the single most important measure for preventing yellow fever. In high risk areas where vaccination coverage is low, prompt recognition and control of outbreaks through immunization is critical to prevent epidemics. Vaccination coverage must reach at least 60% to 80% of the population at risk to prevent outbreaks in affected regions.

Mosquito control can be vital until vaccination takes effect. The risk of yellow fever transmission in urban areas can be reduced by eliminating mosquito breeding sites and applying insecticides to water areas where mosquitos develop.