Emergencies preparedness, response

Yellow fever

Winning the war against yellow fever

WHO/D. Lourenco

25 November 2016 -- Four months have passed without a single case of yellow fever related to the outbreak in Angola and Democratic Republic of the Congo, thanks to the joint response activities of national health authorities, local health workers, WHO and partners.

Resetting the yellow fever strategy

WHO/E. Soteras Jalil

Changes in the way humans live and work, and the resurgence of mosquito vectors, particularly the Aedes aegypti mosquito (which spreads Zika, dengue fever and chikungunya), have raised the global risk of yellow fever. Two large yellow fever outbreaks in Angola and Democratic Republic of the Congo are now under control but these are just warnings of bigger outbreaks to come if action is not taken.

A coalition of partners working to stop yellow fever outbreaks met in Geneva on 12 September 2016 to develop a new strategy - Eliminating Yellow fever Epidemics (EYE). This strategy aims to protect the populations most at risk, ensure a ready supply of yellow fever vaccine, build resilience in urban centres and prevent international spread.

Mass vaccination campaign to protect millions against yellow fever

WHO/E. Soteras Jalil

16 August 2016 -- One of the largest emergency vaccination campaigns ever attempted in Africa started in Angola and Democratic Republic of the Congo this week as WHO and partners work to curb a yellow fever outbreak that has killed more than 400 people and sickened thousands more.

Timeline: Yellow fever outbreak 2016

WHO/M. Marrengula

The outbreaks of yellow fever in Angola, Democratic Republic of the Congo and Uganda have placed great demand on the global supply of yellow fever vaccines. The global stockpile of 6 million vaccines for emergency response (normally enough for a year) has already been replenished twice this year.

This timeline shows the demands on the global vaccine supply since early 2016.

Communication and social mobilization in yellow fever mass vaccination campaigns

WHO/E. Kabambi
Community engagement to mobilise communities to protect themselves against mosquito bites, and to destroy mosquito breeding sites around their homes, forms part of the vaccination campaign activities in DRC.

The ongoing yellow fever outbreak in Angola is mostly in urban areas but at risk of spreading to other provinces and across borders to neighbouring countries. Extensive local transmission in the capital city Luanda, has prompted multiple vaccination campaigns in Angola as well as in Democratic Republic of the Congo and Uganda.

Read WHO’s latest guidance on communication and social mobilization methods to conduct yellow fever vaccination campaigns. The guidance also provides information on the monitoring and evaluation of communication and social mobilization techniques. These 10 points from field experience are especially useful for district-level planning. The document is available in English, French and Portuguese. A Spanish version is also in a pipeline.

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.

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

Vaccine

By mid-June 2016, almost 18 million doses of yellow fever vaccine have been distributed in emergency vaccination campaigns in Angola, Democratic Republic of the Congo, and Uganda.

Geographical distribution

Information in Portuguese

This page links all WHO technical and general information on yellow fever in Portuguese.

General information