Once contracted, the 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 malaria, typhoid, dengue, hepatitis 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.
Forty-four endemic countries in Africa and Latin America, with a combined population of 897 million, are at risk. This counts an estimated 508 million people in 32 African countries, and 12 countries in Latin America, with Bolivia, Brazil, Colombia, Ecuador and Peru at greatest risk.
There are an estimated 200 000 cases of yellow fever (causing 30 000 deaths) worldwide each year. Small numbers of imported cases occur in countries free of yellow fever. Although the disease has never been reported in Asia, the region is at risk because the conditions required for transmission are present there.
The yellow fever virus is an arbovirus of the flavivirus genus, and the mosquito is the primary vector. It carries the virus from one host to another, primarily between monkeys, from monkeys to humans, and from person to person.
Several different species of the Aedes and Haemogogus mosquitoes transmit the virus. The mosquitoes either breed around houses (domestic), in the jungle (wild) or in both habitats (semi-domestic). There are three types of transmission cycles.
There is no specific treatment for yellow fever, only supportive care to treat dehydration 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.
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. To prevent outbreaks throughout affected regions, vaccination coverage must reach at least 60% to 80% of a population at risk. Few countries in Africa currently have this level of coverage.
Preventive vaccination can be offered through routine infant immunization and one-time mass campaigns to increase vaccination coverage in countries at risk, as well as for travelers to yellow fever endemic area. WHO strongly recommends routine yellow fever vaccination for children in areas at risk for the disease.
The yellow fever vaccine is safe and affordable, providing immunization for 99% of those vaccinated within one week. A single dose provides protection for 30–35 years or more, and probably for life. Serious side effects are extremely rare. Serious adverse events have been reported rarely following immunization in a few endemic areas and among vaccinated travelers (e.g. in Brazil, Australia, the United States, Peru and Togo). Scientists are investigating the causes.
The risk of death from yellow fever is far greater than the risks related to the vaccine. People who should not be vaccinated include:
Travelers, particularly those arriving to Asia from Africa or Latin America must have a certificate of yellow fever vaccination. If there are medical grounds for not getting vaccinated, International Health Regulations state that this must be certified by the appropriate authorities.
Mosquito control is vital until vaccination takes effect. The risk of yellow fever transmission in urban areas can be reduced by eliminating potential mosquito breeding sites and applying insecticides to water where they develop in their earliest stages. Application of spray insecticides to kill adult mosquitoes during urban epidemics, combined with emergency vaccination campaigns, can reduce or halt yellow fever transmission, "buying time" for vaccinated populations to build immunity.
Historically, mosquito control campaigns successfully eliminated Aedes aegypti, the urban yellow fever vector, from most mainland countries of central and South America. However, this mosquito species has re-colonized urban areas in the region and poses a renewed risk of urban yellow fever.
Mosquito control programmes targeting wild mosquitoes in forested areas are not practical for preventing jungle (or sylvatic) yellow fever transmission.
Prompt detection of yellow fever and rapid response through emergency vaccination campaigns are essential for controlling outbreaks. However, underreporting is a concern – the true number of cases is estimated to be up to 250 times what is now being reported.
WHO recommends that every at-risk country have at least one national laboratory where basic yellow fever blood tests can be performed. One confirmed case of yellow fever in an unvaccinated population should be considered an outbreak, and a confirmed case in any context must be fully investigated, particularly in any area where most of the population has been vaccinated. Investigation teams must assess and respond to the outbreak with both emergency measures and longer-term immunization plans.
WHO is the Secretariat for the International Coordinating Group for Yellow Fever Vaccine Provision (ICG). The ICG maintains an emergency stockpile of yellow fever vaccines to ensure rapid response to outbreaks in high risk countries.
The Yellow Fever Initiative is a preventive vaccination effort led by WHO and supported by UNICEF and National Governments, with a particular focus on 12 participating African countries where the disease is most prominent. The Initiative recommends including yellow fever vaccines in routine infant immunizations (starting at age 9 months), implementing mass vaccination campaigns in high-risk areas for people in all age groups aged 9 months and older, and maintaining surveillance and outbreak response capacity. Between 2007 and 2009, eight countries have completed preventive yellow fever vaccination campaigns: Benin, Burkina Faso, Cameroon, Liberia, Mali, Senegal, Sierra Leone and Togo. The Yellow Fever Initiative is financially supported by the GAVI Alliance, ECHO, Ministries of Health, and country-level partners.
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