Yellow fever is a serious viral disease that is usually transmitted by a type of mosquito known as the Aedes aegypti mosquito.
Yellow fever mainly occurs in sub-Saharan Africa (countries to the south of the Sahara desert), South America and in parts of the Caribbean. There have not been any recent cases of yellow fever in north America, Europe or Asia.
Yellow fever can be fatal. About 5% of people who get yellow fever die from the condition.
Typical symptoms of yellow fever include:
- high temperature (fever),
- nausea and vomiting,
- jaundice, and
- bleeding (haemorrhage).
Types of yellow fever
There are two types of yellow fever:
Jungle yellow fever
Jungle yellow fever is spread by jungle mosquitoes and mainly affects non-human hosts, such as monkeys. Humans who spend time in a jungle habitat where the mosquito and infected monkeys live can get jungle yellow fever and can be responsible for causing outbreaks of urban yellow fever.
Urban yellow fever
Humans can get urban yellow fever if they are bitten by an Aedes aegypti mosquito that has previously bitten a monkey or human who has the yellow fever infection. The risk of an urban yellow fever outbreak occurring is highest in areas that are close to jungles where the mosquitoes and infected monkeys live.
Cases of both jungle and urban yellow fever most frequently occur in Africa (particularly west Africa). Urban yellow fever is rare in the Americas.
Yellow fever vaccination
The yellow fever vaccination is recommended for:
- anyone travelling to or living in an area or country where yellow fever is a problem, and
- anyone travelling to a country that requires an International Certificate of Vaccination against yellow fever for entry.
See Facts, above, for a list of countries where there is a risk of yellow fever. See Prevention for more details about the yellow fever vaccination and the International Certificate of Vaccination.
The World Health Organization (WHO) has produced a list of the countries and areas where there is an increased risk of yellow fever.
Risk areas in Africa
Most cases of yellow fever occur in sub-Saharan Africa (the countries and regions that are located to the south of the Sahara desert).
In the past, other areas of Africa have also been affected by outbreaks of urban yellow fever.
The areas in Africa where there is a risk of getting yellow fever are:
- Burkina Faso,
- Central African Republic,
- Ivory Coast,
- Democratic Republic of the Congo,
- Equatorial Guinea,
- Guinea Bissau,
- Sao Tome and Principe,
- Sierra Leone,
- Togo, and
Risk areas in South America
The areas in South America where there is a risk of getting yellow fever are:
- French Guiana,
- Suriname, and
Risk areas in the Caribbean
In the Caribbean, there is a risk of getting yellow fever in Trinidad and Tobago.
It is important to remember that in some countries where the number of human cases of yellow fever is low, travellers to that country may still be at risk of infection. For example, a low incidence of yellow fever cases may be the result of a widespread vaccination programme, but the risk of unvaccinated visitors getting the disease may still be present.
The incubation period of yellow fever is usually between three and six days. The incubation period is the time between a person being infected and the onset of symptoms.
Yellow fever can be easily transmitted by infection-carrying mosquitoes to people who are living or travelling in highrisk areas. It cannot be spread by close contact from one person to another.
The symptoms of yellow fever are sometimes categorised into two stages that are known as the acute first stage and the toxic second stage.
Acute first stage
The initial symptoms of yellow fever can include:
- a high temperature (fever) of 38°C (100.4°F) or above,
- chills (shivers),
- nausea and vomiting,
- aching muscles,
- backache, and
- loss of appetite.
Symptoms usually improve after three or four days.
Toxic second stage
After the initial symptoms of yellow fever, about 15% of people then develop more severe symptoms. This is sometimes referred to as the toxic second stage. The symptoms can include:
- a recurrent high temperature (fever) of 38°C (100.4°F) or above,
- lethargy (feeling tired and sluggish),
- jaundice (a yellow tinge to the skin and whites of the eyes caused by liver damage),
- kidney failure, and
- bleeding (haemorrhage).
Between 20-50% of people who develop severe symptoms of yellow fever die within 10-14 days. This means that 3-7.5% of people who develop yellow fever will die from it.
People who recover from yellow fever do not usually have major organ damage and are immune from the disease for life.
Yellow fever is caused by a type of virus known as a flavivirus. The disease is transmitted by the bite of mosquitoes, typically the Aedes aegypti mosquito.
The mosquito becomes infected by biting an animal (or human) that is already infected with the flavivirus. Infected mosquitoes can then pass it on to other animals or humans that they bite. Once infected, a mosquito is a source of infection throughout its life.
The falvivirus is thought to be widespread among monkeys that live in the jungle canopy (the tree tops) of some parts of Africa and the Americas.
Occasionally, an infected mosquito will pass the flavivirus on to a person in the jungle, such as a forestry worker, who may then become a source of infection when they return to their community.
A diagnosis of yellow fever is usually based on a person's symptoms and by carrying out a blood test.
It can sometimes be difficult to make a confident diagnosis of yellow fever based on the symptoms alone, because the symptoms are often similar to a number of other conditions including:
- viral hepatitis, and
- leptospirosis (an infectious disease passed to humans from certain animals).
If you have yellow fever, a blood test will be able to highlight the presence of antibodies that are produced by the body to fight the virus.
The blood test may also show a reduction in the number of infection-fighting white blood cells. This condition is known as leukopenia.
There is no specific anti-viral treatment for yellow fever. However, the symptoms can be treated.
For example, a high temperature (fever) can be treated using non-steroidal anti-inflammatory drugs (NSAIDs), such as paracetamol or ibuprofen. NSAIDs can also be used to treat symptoms such as headaches, back pain and muscle pain.
Children under 16 should not take aspirin. If you have (or have had in the past) gastrointestinal problems, such as stomach ulcers or kidney disease, or if you have cardiovascular (heart) problems, you should not take ibuprofen.
Check with your GP or pharmacist if you're not sure if it's safe for you to take certain types of painkillers.
Immunisation is the most effective way of preventing yellow fever.
Yellow fever vaccination
A single dose of the yellow fever vaccine will protect you against yellow fever for at least 10 years. It is recommended that you have a booster dose every 10 years if you are still at risk of infection.
You must have a yellow fever vaccination at least 10 days before travelling to an 'at risk' country or area. This will allow enough time for the vaccine to become effective by enabling your system to develop immunity against the yellow fever infection.
An International Certificate of Vaccination will become valid 10 days after you have the yellow fever vaccination (see below).
The yellow fever vaccination is safe, but it may cause some side effects. Common side effects include:
- pain and swelling at the site of the injection,
high temperature (fever) of 38°C (100.4°F), or above,
- diarrhoea, and
These side effects are often mild and will disappear after about a week. Rarer side effects of the yellow fever vaccination can include:
- abdominal pain, and
- arthralgia (joint pain).
Seeking medical advice
Always consult your GP if you are planning to travel to an area where there is a risk of getting yellow fever. If you tell your GP where you are travelling to, they can advise you whether you need to be vaccinated against yellow fever and if you require an International Certificate of Vaccination.
International Certificate of Vaccination
Under the Health Regulations set out by the World Health Organization (WHO), anyone travelling to a country or area where the Aedes aegypti mosquito is found must have an International Certificate of Vaccination.
If you have been travelling in an 'at risk' area during the past month, it is recommended that you carry a yellow fever certificate with you. This will help you to avoid potential problems with immigration. For example, it is possible for travellers without a valid yellow fever vaccination certificate to be vaccinated and held in isolation for up to 10 days. .
If you lose your yellow fever vaccination certificate, you may be able to get another one re-issued as long as you have details of the vaccination batch number and the date that you had the vaccination.
Designated vaccination centres
The yellow fever vaccination can only be given at certain designated and registered centres. These may be GP surgeries or travel health centres.
Who should not have the yellow fever vaccination?
People who should not have the yellow fever vaccination are listed below.
- Children under six months of age.
- Children between six and nine months of age should only be immunised if the risk of yellow fever during travel is unavoidable.
- Pregnant women (unless the risk of yellow fever is unavoidable).
- Women who are breastfeeding (unless the risk of yellow fever is unavoidable).
- People whose immune systems are lowered (immunosuppressed), such as those with HIV and people receiving chemotherapy or radiotherapy.
- People who are allergic to eggs (the vaccine contains small amounts of egg).
In cases where having a yellow fever vaccination is not advised (for example, if your immune system is suppressed or if you're pregnant) your GP may be able to issue with you an exemption letter. The letter should be written on headed notepaper, including the practice details, and may be accepted by some immigration authorities.
If you are travelling from an area where there is a risk of yellow fever without a valid yellow fever certificate, immigration officials are legally entitled to quarantine travellers for a period of at least seven days at the point of arrival into a country.