Yellow fever is an acute haemorrhagic infection transmitted by the bite of a mosquito. There is no specific treatment for it. A number of severe outbreaks around the world in the 19th and 20th centuries led to the development of a highly effective vaccine. Systematic vaccination campaigns have considerably reduced yellow fever epidemics but sub-optimal vaccination coverage across several countries in Africa has allowed new outbreaks of the disease to emerge.
Most people only develop a mild form of the disease, with symptoms including fever, headache, muscle pain and nausea. After three or four days, they start to recover. But a small percentage of people enter a second phase that can be fatal.
Yellow fever is transmitted through the daytime bite of infected female Aedes mosquitoes.
Large epidemics occur in regions without good vaccination coverage—often a sign of a failing health system. Outbreak risk is especially high when displaced people move into heavily populated areas with high mosquito density and where the disease is common.
Vaccination is the best way to prevent infection and stop outbreaks. People can also protect themselves against mosquito bites by wearing long clothing and using repellents and insecticide-treated mosquito nets, where available. At the community level, control measures include eliminating stagnant water where mosquitoes breed, and spraying insecticide.
Yellow fever is difficult to diagnose, because many of its mild, early flu-like symptoms resemble those of many other infections.
More severe cases can be confused with severe malaria, leptospirosis, viral hepatitis, other haemorrhagic fevers, infection with other flaviviruses (e.g. dengue fever), and poisoning.
Blood tests can sometimes detect the virus in the early stages of the disease. In later stages, laboratory testing to identify antibodies is needed, which is not readily available in resource-limited settings and in many countries.
Vector control - methods used to limit or eliminate any insect, bird or mammal that transmits a disease pathogen - is essential to preventing infection and reducing transmission. As mosquitoes are the vectors of yellow fever, larvicides are applied to potential breeding sites and insecticides are sprayed to eliminate adult mosquitoes.
Most infections show no symptoms or only mild illness three to six days after infection, with fever and sometimes headache, chills, back pain, fatigue, nausea, and vomiting.
About 15 percent of infections progress to a severe phase, with recurring fever plus jaundice due to liver damage, causing yellow eyes. Some patients develop abnormal bleeding, for example in the mouth, eyes, and gastrointestinal tract. Half of these patients die within 10 days.
There is no specific antiviral drug for yellow fever, but hospital-based supportive care can save the lives of some patients. Surviving the infection provides long-lasting immunity, and typically there is no permanent organ damage.
We have responded to yellow fever epidemics since the year 2000 in countries including Angola, Democratic Republic of Congo, Guinea, Sudan, Sierra Leone, Central African Republic, and Chad. We aim to control outbreaks through mass vaccination campaigns and community-wide insecticide spraying to kill the mosquitoes that spread the disease, and to provide care for people with severe cases. We also work to improve the reach of a limited vaccine stockpile, and to advocate for increased vaccine supply and better routine vaccination coverage.
The largest yellow fever outbreak of the past 30 years began in Angola in December 2015 and spread to neighbouring Democratic Republic of Congo. In response, we participated in a wide-scale vaccination campaign that collectively immunised over 8 million people in less than two weeks. We mobilised 100 teams of 16 people in parts of Kinshasa, the capital, to reach 760,000 people—10 percent of the target population in the city—and another 69 teams that vaccinated the 370,000 residents of Matadi, near the Angolan border. Such rapid, large-scale campaigns come with many logistical challenges, especially to ensure that vaccines are kept cold. Every day, teams needed to replenish 4,000 ice packs and coolers in different locations.
Spraying and fumigating typically targets homes, to kill adult mosquitoes, and trash heaps and stagnant water that provide breeding sites. These activities are carried out in settings where cases have been confirmed and in vulnerable locations such as hospitals, schools, and markets.
During outbreaks, we treat patients directly and also support national and/or local health departments in providing care, through preparing selected hospitals to manage patients and training health staff.
Shortages in yellow fever vaccine supply became especially acute during the major 2015-2016 outbreak. Looking for ways to expand the number of people who could be vaccinated, the World Health Organization and partners conducted a study that showed that the vaccine is effective even when used at one-fifth the usual dose. Using this research, our nurses administered fractional doses of the yellow fever vaccine, significantly expanding the numbers of at-risk people protected through the vaccination campaign. We also support efforts to secure adequate supplies of vaccines by increasing production.
There is no specific antiviral drug for yellow fever, but hospital-based supportive care can save the lives of some patients. Vaccination is the best way to prevent infection and stop outbreaks. We are dependent on private donors like you to provide medical care wherever people’s needs are greatest.
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