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Yellow fever

Yellow fever

6 November 2017

Causative agent

Yellow fever is an acute infection caused by the yellow fever virus. The virus mainly infects monkeys and humans. It is endemic in tropical areas of Africa and Latin America. Large outbreaks of yellow fever were reported in some African countries (e.g. Angola and Democratic Republic of the Congo) and Brazil in 2016.

Clinical features

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, a small percentage 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%.

Mode of transmission

Yellow fever virus is transmitted via the bite of infected mosquitoes. Mosquitoes feed on infected humans or animals mainly monkeys, and then pass on the virus when they feed on other humans or animals.

Incubation period

About 3 – 6 days


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.

Patients carrying the virus should be isolated to prevent mosquito bites that may spread the disease to others.



The World Health Organization (WHO) recommends immunisation for all travellers aged 9 months or above, travelling to and from at-risk areas, unless they are contraindicated for vaccination. Travellers who are vaccinated against yellow fever will be given an International Certificate of Vaccination or Prophylaxis which is valid for life from 10 days after the injection. As it takes 10 days for the vaccine to become effective in providing good protection, adequate time should be allowed for vaccination before departure.

Some countries require proof of yellow fever vaccination as a condition of entry or transit for travellers arriving from certain countries. A list of countries with risk of yellow fever transmission and countries requiring yellow fever vaccination can be found on WHO website at

For more information on travel health, please visit Travel Health Service website of the Department of Health at

General measures on preventing mosquito-borne diseases

  1. Wear loose, light-coloured, long-sleeved tops and trousers, and use DEET-containing insect repellent on exposed parts of the body and clothing.
  2. Take additional preventive measures when engaging in outdoor activities:
  • Avoid using fragrant cosmetics or skin care products
  • Re-apply insect repellents according to instructions
  • Treat clothing and gears (such as tents, bed nets) with permethrin (an insecticide). Do NOT use permethrin directly on skin
  1. Special notes when travelling abroad:
  • If going to affected areas or countries, arrange a consultation with doctor at least 6 weeks before the trip to receive yellow fever vaccination
  • Travellers who return from affected areas and feel unwell, e.g. run a fever, should seek medical advice promptly, and provide travel details to doctor

Help prevent mosquito proliferation

  1. Prevent accumulation of stagnant water
  • Change the water in vases once a week
  • Clear the water in the saucers under potted plants every week
  • Cover water containers tightly
  • Ensure air-conditioner drip trays are free of stagnant water
  • Put all used cans and bottles into covered dustbins
  1. Control vectors and reservoir of the diseases
  • Store food and dispose of garbage properly

Pregnant women and children of 6 months or older can use DEET-containing insect repellent. For details about the use of insect repellents and the key points to be observed, please refer to 'Tips for using insect repellents'.

For more information about control and prevention of mosquito breeding, please visit the website of the Food and Environmental Hygiene Department (FEHD) at