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                                Yellow fever


                                  Yellow fever is an epidemic-prone mosquito-borne vaccine preventable disease that is transmitted to humans by the bites of infected mosquitoes. Yellow fever is caused by an arbovirus (a virus transmitted by vectors such mosquitoes, ticks or other arthropods) transmitted to humans by the bites of infected Aedes and Haemagogus mosquitoes. These day-biting mosquitoes breed around houses (domestic), in forests or jungles (wild), or in both habitats (semi-domestic). Yellow fever is a high-impact high-threat disease, with risk of international spread, which represents a potential threat to global health security.

                                  There are 3 types of transmission cycles. The first is sylvatic (or jungle) yellow fever in which monkeys, which are the primary reservoir of yellow fever, are bitten by wild mosquitoes that pass the virus on to other monkeys and occasionally humans. The second is intermediate yellow fever in which semi-domestic mosquitoes infect both monkeys and people. This is the most common type of outbreak in Africa. The third is urban yellow fever of which large epidemics occur when infected people introduce the virus into heavily populated areas with high mosquito density and where people have little immunity. In these conditions, infected mosquitoes transmit the virus from person to person.

                                  Strong case-based surveillance for yellow-fever can help detect outbreaks early as well as spread to new areas. Occasionally, infected travellers have exported cases to countries that are free of yellow fever. However, the disease can only spread easily if the country it is imported to has mosquito species able to transmit it, specific climatic conditions and the animal reservoir needed to maintain it. To prevent international spread, it is essential that the International Health Regulations (2005) are applied and that travellers to high risk areas present yellow fever vaccination certificates – these certificates are valid for life.

                                  The global Eliminate yellow fever epidemics (EYE) strategy has been developed by a coalition of partners (Gavi, UNICEF and WHO) to face yellow fever’s changing epidemiology, resurgence of mosquitoes, and the increased risk of urban outbreaks and international spread. This global, comprehensive long-term strategy (2017-2026) targets the most vulnerable countries, while addressing global risk, by building resilience in urban centres, and preparedness in areas with potential for outbreaks and ensuring reliable vaccine supply.



                                  Yellow fever can present with a wide range of symptoms and severity. Once contracted, the yellow fever virus incubates in the body for 3–6 days. Many people do not experience symptoms, but when these do occur, the most common are fever, muscle pain with prominent backache, headache, loss of appetite, and nausea or vomiting. In most cases, symptoms disappear after 3–4 days.

                                  A small percentage of patients enter a second, more toxic phase within 24 hours of recovering from initial symptoms. High fever returns and several body systems are affected, usually the liver and kidneys. In this phase people are likely to develop jaundice (yellowing of the skin and eyes, hence the name yellow fever), dark urine and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Half of these patients die within 7–10 days.

                                  Yellow fever is difficult to diagnose, especially during the early stages. More severe cases can be confused with severe malaria, leptospirosis, viral hepatitis (especially fulminant forms), other haemorrhagic fevers, infection with other flaviviruses (such as dengue haemorrhagic fever) and poisoning. Polymerase chain reaction (PCR) testing in blood and urine can sometimes detect the virus in early stages of the disease. In later stages, testing to identify antibodies is needed (IgM).



                                  Yellow fever infections can cause serious illness and can be fatal in severe cases. Early supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever but specific care to treat dehydration, fever, and liver and kidney failure improves outcomes. Associated bacterial infections can be treated with antibiotics. Patients need to stay under mosquito nets during the day to limit the risk of spread to others through bites of mosquitoes.

                                  Vaccination is the single most important measure for preventing yellow fever. The prevention of outbreaks can only be achieved if the majority of the population is immunized. The yellow fever vaccine is safe and affordable, and a single dose provides life-long immunity against the disease. People who recover from yellow fever infection are also protected for life. Read more about the yellow fever vaccine.

                                  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.


                                  with vaccination

                                  Yellow fever can be prevented through vaccination and mosquito control.

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                                  Yellow fever case distribution

                                  Yellow fever outbreaks are difficult to predict as transmission is complex and involves many factors (population immunity, viral transmission dynamics, climate and ecological factors). Since 2016, there has been resurgence of yellow fever outbreaks documented in both the Americas and Africa (Angola, Brazil, Nigeria).

                                  Sporadic cases due to spillover from sylvatic transmission can occur even when population immunity is high. Cases with epidemic potential and outbreaks occur in settings of low population immunity.

                                  The distribution of yellow fever cases is monitored through surveillance to detect outbreaks quickly and support rapid response. Epidemiological updates including yellow fever case distribution in the Americas are available here.


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                                  Technical guidance

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