What is Yellow Fever?
Yellow Fever is an acute infection caused by the yellow fever virus. The incubation period is about 3 to 6 days.
The majority of persons infected with yellow fever virus have no illness or only mild illness. Some people may develop sudden onset of fever, chills, headache, back pain, generalised muscle pain, weakness, fatigue, nausea and vomiting. The condition of most patients improves and their symptoms disappear after 3 – 4 days. Those who recover from yellow fever usually have lasting immunity against subsequent infection. However, about 15% of the symptomatic cases will progress to a more severe form of the disease. The severe form is characterised by high fever, jaundice, bleeding, and eventually shock and failure of multiple organs; in some, infection may be complicated by secondary bacterial infection. Fatality rate among severe cases is about 20 – 50%.
How does it spread?
Yellow Fever is endemic in tropical areas of Africa and Latin America (see map below). It is transmitted via the bite of infected mosquitoes. Mosquitoes feed on infected humans or animals mainly monkeys, and then pass on the infection when they feed on other humans or animals.
How can you prevent it?
Avoiding the bite of mosquito is the first line and the best defense against contracting Yellow Fever. You can take the following precautions in preventing mosquito bites:
- Wear long-sleeved shirts and trousers;
- Rest in air-conditioned or well-screened rooms;
- Use aerosol insecticide indoor and use bed nets if sleeping areas are not air-conditioned or screened;
- Use insect repellents containing DEET on exposed skin and clothing. For details about the use of insect repellents, please refer to Frequently Asked Questions.
- If travelling in endemic rural areas, carry a portable bed net and apply permethrin on it as well as to clothes.
The World Health Organization (WHO) recommends immunisation for all travellers aged 9 months and above, travelling to and from at-risk areas, unless they are contraindicated for vaccination. For those who are contraindicated for vaccination, they should obtain a letter of exemption from the medical practitioner and seek advice from the relevant embassy before travel.
Travellers who are vaccinated against yellow fever will be given an International Certificate of Vaccination or Prophylaxis. Travellers should remember that the certificate is valid only 10 days after the primary injection. The first dose of vaccine takes 10 days to provide good protection, hence adequate time should be allowed for vaccination before departure.
Yellow fever is the only disease specified in the International Health Regulations (2005) (IHR (2005)) for which countries may require proof of vaccination from travellers as a condition of entry under certain circumstances. A list of countries with risk of yellow fever transmission and countries requiring yellow fever vaccination can be found on WHO website at http://www.who.int/ith/2016-ith-annex1.pdf?ua=1.
Following the amendment to the IHR (2005) on 11 July 2016, currently valid certificates continue to be valid for the life of the traveller indicated. Existing certificates do not need to be changed or modified to show they are valid for life.
How is it treated?
There is no specific drug treatment for yellow fever. Management is mainly for symptomatic relief. Associated bacterial infections can be treated with antibiotics. For severe forms of infection, supportive care may include intravenous fluid replacement, dialysis and blood transfusion.
If symptoms of infection occurred, one should consult medical advice as soon as possible. Patients carrying the virus should be isolated to prevent mosquito bites that may spread the disease to others.