Emergencies preparedness, response

Yellow fever

Video: Yellow fever - facts and challenges

How is yellow fever transmitted? Is there a vaccine? Why is an urban yellow fever outbreak such a concern? Dr Sylvie Briand, Director of Pandemic and Epidemic Diseases Department at WHO gives answers to these and other questions in this short, informative video.

WHO Director-General visits Angola; urgent action needed to contain yellow fever outbreak

WHO/J. Caetano

The Director-General of WHO, Dr Margaret Chan, went to Angola’s capital Luanda in the beginning of April 2016 for a two-day visit to assess the situation of the current outbreak of yellow fever virus.

“This is the most serious outbreak of yellow fever that Angola has faced in 30 years,” said Dr Chan, who visited a vaccination point in Luanda. “WHO is taking urgent action to support the Government to control this outbreak with a widespread vaccination campaign.”

Angola grapples with worst yellow fever outbreak in 30 years

WHO/K. Nishino

The yellow fever outbreak, which was first reported in the capital city Luanda in December 2015, has since spread to 6 of the country’s 18 provinces.

WHO has taken urgent action to contain this outbreak, working with the Angolan Ministry of Health and partners to vaccinate people in the affected provinces, using vaccines from the International Coordination Group emergency stockpile.

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).

Occasionally, infected travellers from areas where yellow fever occurs have exported cases to countries that are free of yellow fever, but the disease can only spread easily if that country has mosquito species able to transmit it, specific climatic conditions and the animal reservoir needed to maintain it.

Once contracted, the yellow fever virus incubates in the body for 3 to 6 days, followed by infection that can occur in one or two phases. The first, "acute", phase usually causes fever, muscle pain with prominent backache, headache, shivers, loss of appetite, and nausea or vomiting. Most patients improve and their symptoms disappear after 3 to 4 days.

However, 15% of patients enter a second, more toxic phase within 24 hours of the initial remission. High fever returns and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Once this happens, blood appears in the vomit and faeces. Kidney function 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. It can be confused with severe malaria, dengue hemorrhagic fever, leptospirosis, viral hepatitis (especially the fulminating forms of hepatitis B and D), other hemorrhagic fevers (Bolivian, Argentine and Venezuelan hemorrhagic fevers as well as other Flaviviridae such as the West Nile and Zika viruses) and other diseases, as well as poisoning.

Blood tests can detect yellow fever antibodies produced in response to the infection. Several other techniques are 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, small-scale epidemics occur. Semi-domestic mosquitoes (that breed in the wild and around households) infect both monkeys and people. Increased contact between people and infected mosquitoes leads to transmission. Many separate villages in an area can suffer cases simultaneously. This is the most common type of outbreak in Africa. 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 a high number 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 it is rarely available in poorer areas.

Yellow fever can be prevented through vaccination and mosquito control.

The yellow fever vaccine is safe and affordable, and a single dose provides life-long immunity against the disease.

Mosquito control can also help to prevent yellow fever, and is vital in situations where vaccination coverage is low or the vaccine is not immediately available. Mosquito control includes eliminating sites where mosquitoes can breed, and killing adult mosquitoes and larvae by using insecticides in areas with high mosquito density. Community involvement through activities such as cleaning household drains and covering water containers where mosquitoes can breed is a very important and effective way to control mosquitoes.

Technical information

General information