Increased travelling to Africa and Latin America raises the risks of yellow fever infections here.
Yellow fever is a viral haemorrhagic infection that is common in tropical Africa and Latin America. It is caused by the yellow fever (YF) virus. Its importance lies in its virulence and increasing incidence among travellers to the endemic areas.
Yellow fever is spread by mosquitoes which feed on wild animals infected with the YF virus. These mosquitoes then transfer the infection to people who happen to be in the jungle. On their return to towns or cities, the infected individuals may get bitten by the domestic mosquito Aedes aegypti, which then transmits YF to others.
Following a bite from an infected mosquito, the virus multiplies at the site of the bite and spreads to adjacent lymph nodes, liver, spleen, bone marrow and heart muscle.
The virus is found in the bloodstream during the incubation period of three to six days and the early stages of the illness.
The YF viruses in the bloodstream are spread to another person following ingestion by the blood-feeding Aedes aegypti.
The clinical manifestations of an infection are varied and include flu-like symptoms or febrile illness with jaundice or fatal haemorrhagic disease. YF infection may be mistaken for a number of other infectious diseases such as malaria, dengue and hepatitis.
The onset of illness is abrupt with fever, chills, headaches, backache, muscle aches, nausea, vomiting, congestion of the face and eyes and a slow heart beat. The fever lasts three to four days and is followed by complete recovery in those with the transient non-jaundice type of infection.
However, the illness progresses in about 15%, with or without a break of one to two days, to a more serious variety. There is fever, vomiting, upper abdominal pain, yellowness of the eyes and the skin (jaundice). Liver failure, kidney failure, fits and haemorrhagic tendencies due to decreased production of clotting factors and intravascular coagulation may occur. Coma may also occur.
About 30 to 50% of those with liver and/or kidney failure die a week or so after the onset of illness. Those who survive may have prolonged fatigue and weakness. However, there is complete recovery of their liver and kidney function.
There is no specific treatment available for YF infection. Treatment is only supportive, such as for dehydration and bacterial infection. YF is a very serious illness and public health problem in the endemic areas. There are an estimated 200,000 cases annually with 90% occurring in Africa. About 30% of those infected die.
The World Health Organisation (WHO) estimates that the risk of YF infection to an unvaccinated person entering an endemic area in Africa is one in 267 and the risk of death is one in 1,333. The risks in Latin America are 10 times lower.
Although there are estimated to be more than 10 million travellers to endemic areas, the incidence of imported cases of YF in non-endemic countries is low because of high YF vaccination rate among travellers.
Yellow fever vaccine
The yellow fever vaccine is a live attenuated vaccine, that is, live disease-producing virus modified in the laboratory so that it stimulates the production of antibodies. YF vaccine can be safely given simultaneously with other vaccines like diphtheria-pertussis-tetanus (DPT), polio, measles, hepatitis B, hepatitis A and typhoid.
YF vaccines are recommended for all children aged 12 months and above, older children and adults living in endemic areas. It is also given to individuals who move into an endemic area to stay. When outbreaks of YF occur, mass vaccination is carried out as soon as possible.
YF vaccination is required for all individuals aged nine months and above, who travel to and from endemic countries unless they have medical conditions which make vaccination unsuitable.
Vaccination should be done at least 10 days prior to arrival in the endemic area.
YF vaccination has to be carried out in approved centres with WHO-approved vaccines in order for entry in the International Certificate of Vaccination. This certificate has to be made available when entering or leaving an endemic country.
Safety and effectiveness
The YF vaccine is very effective. Protective antibody levels are found in 90% of vaccinees within 10 days and 99% within a month. The protection appears to last for 30 to 35 years. However, there is insufficient evidence to support a change in the International Health Regulations for travellers to endemic areas to show proof of valid YF vaccination within the preceding 10 years.
YF vaccines are well tolerated although mild reactions such as headache, muscle ache and weakness can occur within a few days after vaccination, in 10 to 30% of vaccinees. Serious adverse reactions are very rare. They include hypersensitivity reactions, vaccine associated neurological disease and vaccine associated viscerotropic disease.
Hypersensitivity reactions are usually due to reaction to the eggs used in the production of the vaccine and rarely, gelatine, which is used to stabilise the vaccine.
Vaccination is not given to infants below nine months of age, individuals with severe allergy to eggs, pregnant women and severely immunocompromised individuals. However, the YF vaccine may be given to pregnant women and infants during an epidemic.
Yellow fever is not endemic in Malaysia but the mosquito carrier of YF is commonly found here. Increased travelling to and from endemic areas in African and Latin America increases the likelihood of YF being introduced into the country. Vaccination is the single most important intervention to control yellow fever.
