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Yellow fever

Yellow fever

10 December 2019

Causative agent

Yellow fever is an acute infection caused by the yellow fever virus, which mainly infects monkeys and humans. It is endemic in tropical areas of Africa and Central and South America. In recent few years, yellow fever cases have been reported in many countries in Africa and the Americas with major outbreaks in Brazil, Congo, Ethiopia, Nigeria and South Sudan.

Clinical features

The majority of persons infected with yellow fever virus have no or only mild illness. Some people may develop fever, muscle pain (especially backache), headache, loss of appetite, and nausea or vomiting. The condition of most patients improves and their symptoms disappear after 3 to 4 days. Those who recover from yellow fever usually have lasting immunity against subsequent infection. However, a small percentage of the cases will progress to a more severe form of the disease (characterised by high fever, jaundice, bleeding, and shock and failure of multiple organs). Some cases may be complicated by secondary bacterial infection. Fatality rate among severe cases is about 30 to 60%.

Mode of transmission

Yellow fever virus is transmitted via the bite of infected Aedes mosquitoes (mainly Aedes aegypti). 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 to 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 (except those who are contraindicated) immunisation for all travellers aged 9 months or above, travelling to and from at-risk areas. In Hong Kong, the service is provided at the travel health centres of the Department of Health (DH). It takes 10 days for the vaccine to become effective in providing protection. Travellers should be aware of the risk of yellow fever when planning to travel to affected areas and arrange travel health consultation at least 8 weeks before departure. 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.

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 the WHO website at

For more information on travel health, please visit Travel Health Service website of the DH 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:
  • During the trip, if travelling in endemic rural areas, carry a portable bed net and apply permethrin on it. Travellers who return from affected areas and feel unwell e.g. run a fever, should seek medical advice promptly, and provide travel details to the doctor.

Help prevent mosquito proliferation

  1. Prevent accumulation of stagnant water
  • Change the water in vases once a week
  • Avoid using saucers underneath flower pots
  • 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 children who travel to countries or areas where mosquito-borne diseases are endemic or epidemic and where exposure is likely, children aged 2 months or above can use DEET-containing insect repellents with a concentration of DEET up to 30%. 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