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Diseases / Yellow Fever

Yellow Fever

What is it?: 

Yellow Fever (YF) is a mosquito-borne viral disease of humans and other primates, and is currently endemic in 44 countries in the tropical regions of Africa and South America. According to WHO estimates from the early 1990s, 200,000 cases of YF, with 30,000 deaths, are expected globally each year, with 90% occurring in Africa. Infection with the YF virus can be asymptomatic or cause a wide spectrum of disease, from mild symptoms to severe illness with bleeding, jaundice and, ultimately, death.1

One study has estimated that unvaccinated travelers who visit areas in Africa during periods of epidemic activity have a 1 in 267 risk of illness and a 1 in 1333 risk of death due to YF. However, travellers’ risk of acquiring YF is difficult to predict due to variations in the ecologic determinants of virus transmission and in protective behaviours, immunity profiles, and activities.1

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%.2

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.2

Incubation period: 

About 3 – 6 days.2



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


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.2

Patients carrying the virus should be isolated to prevent mosquito bites that may spread the disease to others.2


  1. World Health Organization. Yellow fever vaccines: WHO position paper. Jul 2013.
  2. Centre for Health Protection. Communicable diseases – Yellow Fever. 10 December 2019. [ONLINE] Accessed on 12 Apr 2021.