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Yellow fever

Introduction

Yellow fever is a serious viral disease that's usually spread by a type of mosquitoe, known as the Aedes aegypti mosquitoIt can be prevented with a vaccination (see below).

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 haven't been any recent cases of yellow fever in North America, Europe or Asia. Since 1996, six European or US travellers have died from yellow fever. None of them were vaccinated. 

Read more about the risk areas for yellow fever.

Yellow fever can be fatal. About 7 or 8 people out of 100 who get yellow fever die from it.

Typical symptoms of yellow fever include:

  • headache
  • high temperature
  • nausea and vomiting
  • jaundice – yellowing of the skin and whites of the eyes
  • bleeding

How yellow fever is spread

Yellow fever is a type of virus known as a flavivirus. The virus is transmitted through mosquito bites. It can be spread from one host to another, usually between monkeys, or from monkeys to humans, or from person to person. The mosquitoes live and breed in jungle and urban areas.

See Yellow fever - causes for more information about how yellow fever is transmitted.

Yellow fever vaccination

There's a vaccination for yellow fever. Some countries require a proof of vaccination (certificate) against yellow fever before they let you enter the country.

You should have a yellow fever vaccination at least 10 days before your travel. This will allow enough time for your body to develop protection against the yellow fever infection.

The yellow fever vaccination is recommended for:

  • laboratory workers who handle infected material
  • anyone travelling to, or living in, an area or country where yellow fever is a problem (see Yellow fever - risk areas for a list of countries where there's a risk of yellow fever)
  • anyone travelling to a country that requires an International Certificate of Vaccination or Prophylaxis (ICVP) against yellow fever for entry

See Yellow fever - prevention for more information about the yellow fever vaccination and the vaccination certificate, plus where to get vaccinated.

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Facts

The World Health Organization (WHO) has produced a list of the countries and areas where there is an increased risk of yellow fever.

In some countries, although the number of cases of yellow fever may be low, visitors may still be at risk of infection. For example, a low incidence of yellow fever cases may be the result of a vaccination programme, but there's still a risk of unvaccinated visitors getting the disease. See Yellow fever - prevention for more details about the yellow fever vaccination.

Risk areas in Africa

Most cases of yellow fever occur in sub-Saharan Africa (the countries and regions south of the Sahara desert). In the past, other areas of Africa have also been affected by outbreaks of urban yellow fever.

The areas of Africa where there's a risk of getting yellow fever are:

  • Angola
  • Benin
  • Burkina Faso
  • Burundi
  • Cameroon
  • Central African Republic
  • Chad
  • Congo
  • Ivory Coast (Côte d’Ivoire)
  • Democratic Republic of the Congo
  • Equatorial Guinea
  • Ethiopia
  • Gabon
  • Gambia
  • Ghana
  • Guinea
  • Guinea Bissau
  • Kenya
  • Liberia
  • Mali
  • Mauritania
  • Niger
  • Nigeria
  • Rwanda
  • Sao Tome and Principe
  • Senegal
  • Sierra Leone
  • Somalia
  • Sudan
  • Tanzania
  • Togo
  • Uganda

Risk areas in South America

The areas of South America where there's a risk of getting yellow fever are:

  • Argentina
  • Bolivia
  • Brazil
  • Colombia
  • Ecuador
  • French Guiana
  • Guyana
  • Panama
  • Paraguay
  • Peru
  • Suriname
  • Venezuela

Risk areas in the Caribbean

In the Caribbean, there's a risk of getting yellow fever in Trinidad and Tobago.

Getting up-to-date advice

Over time, the areas that are considered to be of high risk for any disease can change. For up-to-date travel information you can check the following websites:

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Symptoms

The time between being infected with yellow fever and the start of symptoms is usually three to six days.  This time is called the incubation period.

The symptoms of yellow fever usually happen in two stages. The initial symptoms (doctors sometimes refer to the acute first stage) can include:

  • a high temperature (fever) of 38ºC (100.4ºF) or above
  • headache
  • chills (shivers)
  • muscle aches 
  • nausea and vomiting
  • backache
  • loss of appetite

Three or four days later these symptoms normally improve.

Second stage

After the initial symptoms of yellow fever, around 15% of people who have been infected with this virus then develop more severe symptoms.  This is sometimes referred to by doctors as the toxic second phase.  The symptoms can include:

  • a recurrent high temperature
  • a yellow tinge to the skin and whites of the eyes (jaundice, as a result of damage to the liver)
  • abdominal pain
  • vomiting
  • bleeding can occur from the mouth, nose, eyes or stomach – blood can be seen in the vomit and stools (faeces).
  • kidney failure

Half of the people who have the second phase of yellow fever die within ten to fourteen days. Those who recover with no major organ damage and are then immune to the disease for life.

Overall, this means that around 7 or 8 people out of every 100 who develop yellow fever will die from it.

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Causes

Yellow fever is caused by a type of virus known as a flavivirus.  The disease is transmitted by the bite of certain mosquitoes (see below). 

Yellow fever can't be spread by close contact between two people.

How yellow fever is spread

In urban and some rural areas, it's usually the Aedes aegypti mosquito that spreads yellow fever. However, other types of mosquitoes may carry the virus in forested areas. The mosquitoes usually bite during daylight hours.

The mosquito becomes infected by biting a monkey or human that is already infected with the virus.  Infected mosquitoes can then pass the disease on to any other monkeys or humans that they bite.  Once infected, a mosquito is a source of infection throughout its life.

It is thought that the virus is widespread among monkeys that live in the the jungle canopy (the tree tops) of some parts of Africa and the Americas. 

Occasionally, an infected mosquito will pass the virus 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.

The risk of an urban yellow fever outbreak is highest in areas close to jungles where the mosquitoes and infected monkeys live.

Who's at risk?

If you're travelling, your risk of getting yellow fever may depend on:

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Diagnosis

A diagnosis of yellow fever is usually made based on your symptoms and by carrying out a blood test.

It can sometimes be difficult to diagnose yellow fever based on the symptoms alone because the symptoms are often similar to a number of other conditions, including:

  • malaria – a tropical disease that's spread by night-biting mosquitoes
  • typhoid fever – a serious and potentially fatal bacterial infection
  • viral hepatitis – inflammation of the liver that's caused by a virus
  • leptospirosis – a bacterial infection that's spread to humans from some animals, such as rats
  • Dengue fever – an infectious disease that's also transmitted by the Aedes aegypti mosquito

Blood test

If you have yellow fever, a blood test will be able to detect the presence of special proteins called antibodies, which 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 (leukopenia). This can happen because the yellow fever virus affects your bone marrow, which is the spongy material that's found in the hollow centres of some bones and produces blood cells.

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Treatment

There is no specific antiviral treatment for yellow fever. However, the symptoms can be treated.

For example, a high temperature (fever) can be treated using paracetamol or ibuprofen.  These can also be used to control symptoms such as:

  • headaches
  • back pain
  • muscle pain

See the Health topics about paracetamol and ibuprofen for more information. Children under 16 shouldn't take aspirin.

You should also make sure that you're drinking enough fluids to avoid dehydration.

Treatment in hospital

If your symptoms are severe, you may be admitted to hospital so that you can be monitored and receive supportive nursing care. Additional treatment may be necessary, including:

  • a ventilator to help you breathe
  • a transfusion of red blood cells to replace those lost through bleeding
  • a machine to filter your blood if your kidneys are no longer working (dialysis)
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Prevention

The best way to prevent yellow fever is to have the vaccination.

Yellow fever vaccination

In the UK, Stamaril (Sanofi Pasteur MSD) is the only licensed yellow fever vaccination. A single dose of the yellow fever vaccine will protect you against yellow fever for at least 10 years. It's recommended that you have a booster dose every 10 years if you're still at risk of infection.

Who should be vaccinated?

You should receive the yellow fever vaccination if:

  • you're a laboratory worker and you handle infected material
  • you're travelling to a country that requires you to have an International Certificate of Vaccination or Prophylaxis (ICVP) before letting you into the country; the certificate proves that you've been vaccinated against yellow fever
  • you're travelling to, or living in, an area or country where yellow fever is found – see Yellow fever - risk areas for a list of these countries

You must have a yellow fever vaccination at least ten days before travelling.  This will allow enough time for your body to develop protection against the yellow fever infection.

Your certificate will only become valid 10 days after you have the yellow fever vaccination.

Where do I get vaccinated?

The yellow fever vaccination can only be given at designated and registered centres. For a centre to become a designated yellow fever vaccination centre, it must register with the appropriate authority, which in the UK is either:

  • the National Travel Health Network and Centre (NaTHNac)
  • Health Protection Scotland (HPS)

See the NaTHNaC website to find your nearest yellow fever vaccination centre.

Certificate of proof

Under 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 a Certificate of Vaccination or Prophylaxis (ICVP).

You can find a list of all the countries that require you to have an ICVP in the WHO International travel and health guide. You can also search the country information on NATHNaC to find out if the country you're visiting requires an ICVP. 

If you've been travelling in an ‘at risk’ area during the past month, it might be a good idea to carry your certificate with you. This will help to avoid potential problems with immigration. It's possible for travellers without a valid yellow fever vaccination certificate to be vaccinated and held in isolation for up to 10 days. An ICVP isn't required for entry into the UK.

If you lose your 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.

Seeking medical advice

Always consult staff at a designated vaccination centre if you're planning to travel to an area where there's a risk of getting yellow fever. If you tell them where you're travelling to, they will be able to advise you about whether you need to be vaccinated against yellow fever and whether you need an ICVP.

Who should not be vaccinated

People who should not have a vaccination for yellow fever include:

  • babies under nine months old - babies who are six to nine months old should only be vaccinated if the risk of yellow fever during travel is unavoidable
  • women who are pregnant - unless the risk of yellow fever is unavoidable
  • women who are breastfeeding – unless the risk of yellow fever is unavoidable
  • those with a lowered immune system (immunosuppressed), for example those with HIV receiving chemotherapy or radiotherapy 
  • those with an egg allergy (the vaccine contains small amounts of egg)
  • people who've had a severe allergic reaction (anaphylaxis) to a previous dose of the yellow fever vaccine
  • people who are allergic to any of the ingredients in the vaccine
  • people who have a condition affecting the thymus gland (part of your immune system, which is located in your upper chest)
  • people who are currently severely unwell (e.g. with a high fever) – this is to avoid confusing the diagnosis of your current illness with any side effects from the vaccine
  • yellow fever naive travellers (those who haven't been previously exposed to the vaccine) who are 60 years old or over (unless the risk of yellow fever is unavoidable)

Exemption letters

In cases where having a yellow fever vaccination is not advised, your GP may be able to issue with you an exemption letter. The letter should be written on headed notepaper, including the practice details. It may be accepted by some immigration authorities.

If you're travelling from an area where there's 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.

Side effects of the vaccine

After having the yellow fever vaccine, 10-30% of people will have mild side effects, such as:

  • headache
  • muscle pain
  • soreness at the injection site
  • mild fever

Reactions at the injection site usually occur one to five days after being vaccinated, although other side effects may last for up to two weeks.

An allergic reaction to the vaccine occurs in one case out of every 130,000 doses of the vaccine that are given. 

Yellow fever vaccine-associated neurological disease (YEL-AND)

Rarely, the yellow fever vaccine is associated with a neurological disease, known as yellow fever vaccine-associated neurological disease (YEL-AND). Neurological means that it affects the nerves and the nervous system, including the brain and spinal cord.

YEL-AND occurs in around four cases out of every 1 million doses given. However, for people who are 60 years old or over and are yellow fever vaccine naïve, the incidence of YEL-AND increases to around 1 in every 50,000. This represents the highest risk for any vaccine currently in use.

The symptoms of YEL-AND include:

  • a high temperature (fever) of 38ºC (100.4ºF) or above
  • headache
  • confusion
  • problems with your nerves – for example, a problem with the nerves in your tongue, which affects your ability to speak (focal neurological deficit)
  • coma
  • inflammation of the network of nerves that control the body's senses and movements (Guillain-Barré syndrome)  

Yellow fever vaccine-associated viscerotropic disease (YEL-AVD)

The yellow fever vaccine is also associated with a viscerotropic disease that's known as yellow fever vaccine-associated viscerotropic disease (YEL-AVD). Viscerotropic means that it affects the viscera (your internal body organs, such as the heart or lungs).

YEL-AVD occurs in around three cases out of every 1 million vaccines that are given. However, for people who are 60 years old or over and are yellow fever vaccine naïve, the incidence of YEL-AVD increases to just over 1 in every 50,000. This represents the highest risk for any vaccine currently in use.

Symptoms of YEL-AVD include:

Avoiding mosquito bites

As well as getting vaccinated, avoiding mosquito bites will help to prevent yellow fever. The mosquitoes that carry yellow fever bite during daylight hours. Although it's not always possible, you can try to follow some of the advice listed below.

  • Avoid places where mosquitoes live, such as swamps, forests and jungles.
  • Stay in an air-conditioned room.
  • Stay in a room with mesh screening over the windows and doors.
  • Wear loose fitting, long-sleeved tops and trousers.
  • Spray insect repellent containing DEET onto exposed skin.
  • Burn a mosquito coil or use a plug-in device that releases insecticide.
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