Yellow fever

Yellow fever is a viral infection carried by mosquitoes. It exists in only two parts of the world: tropical Africa and the tropics of South and Central America. Most people who catch it go through something like a bad bout of flu and recover within days, but in a minority the illness moves into a second, far more dangerous phase with jaundice and bleeding. There is no medicine that kills the virus. There is, however, a vaccine that protects for practically a lifetime after a single dose.

Where you catch it

The virus enters the bloodstream through a mosquito bite. It does not pass directly from person to person through coughing or touch.

The risk areas are the tropical belt of Africa south of the Sahara and part of South and Central America: the Amazon basin, the interior of Brazil, Peru, Bolivia and Colombia, plus Trinidad and Tobago. Europe, Asia, Australia and Oceania have no yellow fever, although imported cases in unvaccinated travellers are reported every year.

One detail matters a great deal: the mosquitoes that spread the virus in towns bite during the day, peaking after sunrise and again before sunset. The familiar logic of "a net at night and all is well" does not apply here.

How the illness unfolds

Three to six days usually pass between the bite and the first symptoms. The onset is abrupt: fever climbs within hours, joined by headache, muscle pain especially in the lower back, chills, nausea, vomiting and complete loss of appetite. Sometimes there is a striking combination — high fever with a slow pulse.

After three or four days the fever subsides and the person feels distinctly better. For most, that is where it ends. But in some the fever returns a day after this apparent recovery, and what follows is a different illness in severity: the virus attacks the liver and kidneys, jaundice appears (hence the name), clotting is disrupted and bleeding begins. Of those who reach this phase, roughly half die.

There is no way to predict who will take the severe course. That is why the fever coming back after "recovery" is an alarm signal.

When hospital cannot wait

If you are in tropical Africa or South America, or have recently returned, seek emergency care immediately at any of these signs:

• the skin or the whites of the eyes have turned yellow;
• urine has darkened to the colour of strong tea, or there is noticeably less of it;
• the fever has come back after you started to improve;
• bleeding has appeared — from the nose or gums, in vomit (looking like coffee grounds) or in the stool (black and tarry);
• severe pain under the right ribs, vomiting that will not stop;
• confusion, unusual drowsiness, seizures, fainting.

These are not symptoms to sit on until morning or until the trip ends. Severe yellow fever can only be treated in hospital, and the outcome depends on how quickly you get there. Always tell the doctor which country you were in and when you came back: without that, the diagnosis will be looked for in the wrong place.

What helps and what can do harm

There is no antiviral drug for yellow fever. Treatment means supporting the body while it fights: fluids by mouth or by drip for dehydration, bringing the temperature down, rest, and in severe cases transfusion of blood products and dialysis. Fever and pain are managed with paracetamol at ordinary doses.

And here is what should not be done: in yellow fever, and in any unexplained fever brought back from the tropics, aspirin and anti-inflammatory painkillers such as ibuprofen, naproxen and diclofenac are avoided. They interfere with the work of the platelets and irritate the stomach lining, while clotting is already disturbed by the illness itself — in other words, they raise the risk of exactly the bleeding that kills in the severe form. The familiar advice of "paracetamol or ibuprofen" from articles about colds is wrong for this disease.

Antibiotics have no effect on the virus: they are given only if a bacterial infection has been added on top.

The vaccine and the international certificate

The vaccine is live and attenuated, given as a single injection in the upper arm. Protection takes about ten days to build, so you need to be vaccinated at least ten days before departure. One dose lasts a lifetime for the great majority: routine booster doses are no longer required. Vaccination is offered to anyone travelling to a country where the virus circulates, from nine months of age.

The document issued afterwards is the international certificate of vaccination. It is valid for life, including older forms printed with a ten-year expiry: that note has long ceased to mean anything. Only an authorised centre may issue it, which is why this vaccine is not given at an ordinary clinic. Some countries require the certificate on entry, sometimes only from people arriving from an endemic country, including in transit; requirements change, so check them shortly before travelling.

Who cannot have it, and who needs a careful decision

A live vaccine is not suitable for everyone. It is not given:

• to infants under six months;
• to people with severely weakened immunity — leukaemia and lymphoma, chemotherapy, high doses of systemic steroids, organ transplantation;
• in disease of the thymus gland or after its removal;
• with severe allergy to any component, including egg protein, or a severe reaction to a previous dose of this vaccine.

Vaccination is weighed up separately in pregnancy, in women breastfeeding an infant under nine months, in children aged six to nine months, in people over sixty and in people living with HIV whose immune status is preserved: the decision is individual, and the vaccine is given if the trip cannot be cancelled.

Ordinary after-effects are mild — headache, muscle ache, a slight temperature, soreness at the injection site — and pass within a few days. Very rarely the vaccine causes serious damage to the nervous system or internal organs; the risk is higher past sixty, with a weakened immune system and with thymus disease. If you feel sharply worse after vaccination, with fever, confusion or jaundice, seek advice at once.

Where the vaccine is contraindicated but a country demands the certificate, a vaccination centre can issue a written medical exemption. It is not accepted everywhere, so ask for it well in advance.

Keeping the mosquito off you

The vaccine protects against yellow fever but not against dengue, chikungunya, Zika or malaria, which the same or neighbouring mosquito species carry. Bite protection is needed even after vaccination.

• During the day wear light-coloured, loose clothing with long sleeves and long trousers.
• Put repellent on exposed skin — containing diethyltoluamide, icaridin or oil of lemon eucalyptus — and reapply as instructed.
• Clothing and gear can be treated with permethrin, which is never applied to skin.
• Sleep under an insecticide-treated net, including during a daytime nap, and keep doors and windows shut or screened.
• Do not leave standing water near where you stay: buckets, plant saucers, tyres. That is where mosquitoes breed.

Online consultation: before the trip and after it

Before you leave, an Oladoctor doctor can work through the questions by video call: whether your itinerary carries a yellow fever risk, whether a certificate will be asked for, when to schedule the vaccine, and whether your chronic conditions or medicines complicate it. After you return, an appointment is worth booking if a fever appears within a month of the trip, or if unusual symptoms persist after vaccination.

The online format does not replace emergency care. With jaundice, bleeding, dark urine or fever returning after an improvement, go to hospital rather than wait for a written reply.

This material is for information only and does not replace medical advice.