Yellow Fever is a viral infection caused by RNA viruses belonging to the Flavivirus genus. It is a zoonosis – an animal disease that can spread to humans – primarily transmitted by daytime biting Aedes Aegypti female mosquitoes, but also by mosquitoes belonging to the Haemagogus genus.
In the sylvatic cycle, Yellow Fever is transmitted by mosquitoes that bite infected monkeys passing the infection to humans living in or visiting jungle areas. Yellow Fever is endemic in the sylvatic setting in sub-Saharan Africa and the tropical regions of South America. In the intermediate or savannah cycle, the infection is transmitted to humans via mosquitoes that bite infected monkeys or other humans living or working in jungle border areas in Africa. In the urban cycle, infected mosquitoes transmit Yellow Fever from person to person that can cause large outbreaks in cities and suburbs. Yellow Fever outbreaks occur periodically in Africa and have occurred sporadically in South America.
Travellers are at risk when going to endemic areas of Africa and South America.
Most infections are asymptomatic – persons do not exhibit symptoms. Those with symptoms get ill between 3 to 6 days after exposure to the virus. The infection is characterized by flu-like symptoms, including sudden fever, headache, muscle pain, backache, nausea, and vomiting. Most people will recover from this illness but about 15% will develop severe symptoms within 48 hours, including vomiting, jaundice, bleeding, shock, and multiple organ failure. Treatment includes supportive care of symptoms, but the use of aspirin and non-steroidal anti-inflammatory drugs (NSAIDs) must be avoided since they increase the risk of bleeding.
Yellow Fever is a vaccine preventable disease. Vaccination is recommended for persons over 9 months of age travelling to or living in endemic areas. The vaccine affords long term protection.
Note that some countries require proof of Yellow Fever vaccination for entry (see list below). The World Health Organization announced that as of 11 July 2016, existing and new Yellow Fever vaccination certificates are valid for life starting 10 days after vaccination. Countries can no longer require travellers to show proof of re-vaccination or a booster dose as a condition of entry. Travellers should note that this new regulation may not be honoured by all border authorities during the transition phase.
Vaccination precautions: If travel to an endemic area cannot be delayed or avoided, the following travellers should get vaccinated depending on a risk assessment of Yellow Fever infection at their destination and a country's entry requirement: Children between the ages of 6 to 8 months, persons over 60 years, those with asymptomatic HIV, pregnant or breastfeeding women.
Vaccination contraindications: Children under 6 months of age, persons with immune deficiencies or on immunosuppressive or immunomodulatory therapies, persons with allergies to egg proteins, thymus disorders, malignant neoplasms, transplant recipients, and persons with symptomatic HIV infection should not get the Yellow Fever vaccine. If vaccination is contraindicated for medical reasons an exemption letter or waiver should be issued to the traveller. However, acceptance of such a letter is at the discretion of the destination country, and entry might be denied.
If going to low risk Yellow Fever areas, travellers should take meticulous measures to prevent mosquito bites during the daytime.
Yellow Fever virus images, life cycle, and distribution maps: Infection Landscapes
Information last updated: February 23, 2021.