Yellow fever: symptoms, how it spreads, vaccination and treatment

Yellow fever is caused by a virus that circulates between infected monkeys or humans and mosquitoes. TravelHealthPro, the travel health service run by the National Travel Health Network and Centre, says the virus can cause an illness that results in jaundice (yellowing of the skin and eyes) and bleeding, with severe damage to major organs such as the liver, kidneys and heart. The name comes from the jaundice.

The virus is carried by mosquitoes. TravelHealthPro describes a forest cycle, in which mosquitoes feed on monkeys and occasionally bite people working or living nearby, and an urban cycle, where infected people can introduce the virus to places with dense populations and a mosquito species that breeds around human homes. It notes that these mosquitoes are active in daylight, biting from dawn to dusk.

Infection in a person who recovers results in lifelong immunity, according to the same source. The vaccine is described as a live, attenuated (weakened) virus.

The NHS says some yellow fever symptoms are similar to flu and often last 3 or 4 days. A small number of people then become more seriously ill within 24 hours of feeling better, and these more serious symptoms can be fatal.

TravelHealthPro puts numbers on this phase. It says that within 24 hours of an apparent recovery, 15 to 25 per cent of patients progress to a more serious illness with bleeding, pronounced jaundice and kidney damage, and that 20 to 50 per cent of those who develop this form of the disease do not survive.

• Early symptoms the NHS lists: high temperature, headache, feeling or being sick, aches and pains, loss of appetite and feeling generally unwell.

• More serious symptoms: yellowing of the skin and eyes (jaundice), dark pee, stomach pain.

• Bleeding from the eyes, nose, mouth or stomach, which may show as blood in vomit or poo.

Yellow fever is spread by the bite of an infected mosquito, not by everyday contact. The NHS says you can get it if you are bitten by an infected mosquito, and that the mosquitoes that carry the virus bite during the day. TravelHealthPro says only female mosquitoes of the species involved transmit the virus.

People can be part of the cycle, though. TravelHealthPro describes an urban cycle in which infected people can introduce the virus into places with a dense population, and virus transmission then occurs between humans through mosquitoes that breed around human habitation. It also notes that once infected, a mosquito remains infectious for life.

The NHS says yellow fever is not found in the UK. If you become unwell after travel to a country where it is found, tell anyone you speak to about your travel and whether you were bitten by a mosquito or might have been.

TravelHealthPro says there is a risk of yellow fever transmission in parts of the tropical and sub-tropical regions of Africa and South and Central America, and in Trinidad in the Caribbean. The NHS says the disease is very common in certain parts of the world, including parts of sub-Saharan Africa, South America and Central America.

For travellers, TravelHealthPro lists the factors that determine risk: the destination, how intense transmission is in the area, the season of travel, the length of the trip, activities that expose you to mosquitoes, and your immunisation status. It adds that the disease is preventable by vaccination and remains a very rare cause of illness in travellers.

Because areas of risk change, the country pages on TravelHealthPro are the place to check before you go. The NHS also points people to that site for destination-specific advice.

There is no specific treatment for yellow fever, according to the NHS. Most people make a full recovery after 3 or 4 days, and things you can do to ease your symptoms include taking painkillers such as paracetamol or ibuprofen, which can help lower your temperature and relieve muscle pain or backache, and drinking plenty of fluids to avoid dehydration.

If you have more serious symptoms of yellow fever, the NHS says you will need to go into hospital immediately to be treated. TravelHealthPro says that care of an infected person is based on managing the symptoms, and that when a clinician suspects yellow fever in a returned traveller, an urgent referral to an infectious disease or tropical disease clinical team is required.

TravelHealthPro adds that the UK Health Security Agency Rare and Imported Pathogens Laboratory is a specialist centre providing advice and diagnosis of unusual infections including yellow fever. The preliminary diagnosis is based on the patient's clinical features, the location and dates of travel, and activities.

The NHS says symptoms usually start 3 to 6 days after being bitten by an infected mosquito, but sometimes they can take longer. The flu-like symptoms often last 3 or 4 days, and the NHS says most people make a full recovery after 3 or 4 days.

A small number of people get more serious symptoms within 24 hours of feeling better. TravelHealthPro describes this as an acute haemorrhagic fever with bleeding, pronounced jaundice and kidney damage, in which shock and deterioration of major organ function can occur. It says infection results in lifelong immunity in those who recover.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you feel unwell after travelling to a country where yellow fever is found. Tell anyone you speak to about your recent travel and whether you were bitten by a mosquito or might have been.

It adds that if you become unwell while abroad you should get medical help as soon as possible rather than waiting until you return to the UK. TravelHealthPro says that when a clinician suspects yellow fever in a returned traveller, urgent referral to an infectious disease or tropical disease team is required, with specialist laboratory support available through the UK Health Security Agency.

The NHS says the yellow fever vaccine is recommended if you are travelling to an area where yellow fever is found or if you need a certificate to prove you have been vaccinated. It is safe for most people who are 9 months old or over. It must be paid for when needed for travel and can only be given at registered yellow fever vaccination centres.

TravelHealthPro describes one licensed vaccine in the UK, Stamaril, given as a single dose that provides protection for at least 35 years and may be lifelong, with a minimum age of 9 months. It says that certificate requirements are not necessarily connected to the actual risk of disease for a traveller, and that proof of vaccination under the International Health Regulations may be an entry requirement for some countries.

Not everyone should have the vaccine. TravelHealthPro says people aged 60 or older should only be given it when there is significant and unavoidable risk, and that a careful assessment balancing the risk of disease against the risk of serious vaccine side effects is needed. Where there is a medical reason not to vaccinate, a Medical Letter of Exemption can be provided for entry purposes.

TravelHealthPro lists groups for whom a careful individual risk assessment is needed before the vaccine is given. These precaution groups include pregnant women, breastfeeding women, infants aged 6 to 8 months, people aged 60 and older, people taking low dose steroids or non-biological oral immune modulating drugs, and people living with HIV with a CD4 count above 200 and a suppressed viral load.

It says people aged 60 or older should only be given the vaccine when there is significant and unavoidable risk of acquiring yellow fever, and that the risk of the disease at the destination has to be weighed against the risk of serious vaccine side effects. A Medical Letter of Exemption can be provided where there is a medical reason not to vaccinate and a country requires a certificate.

Because these decisions are individual, ask a yellow fever vaccination centre or your GP well before travel. The NHS says the vaccine can only be given at registered yellow fever vaccination centres.

The NHS lists simple measures for people travelling to an area where yellow fever is found. They matter even if you have been vaccinated, because mosquitoes carry other diseases too.

• Wear long-sleeved clothing and trousers, particularly in the early morning and early evening.

• Use insect repellent on the skin, ideally one that contains DEET.

• Close windows and doors whenever possible, or use blinds or screens.

• Sleep under an insecticide-treated mosquito net, including during the day.