Yellow fever: know the facts

Without a yellow fever vaccination certificate, you'll be denied entry to some countries - but what is yellow fever, and what do you need to know about the vaccine?

How dangerous is yellow fever?

Yellow fever is a virus spread by day-active Aedes mosquitoes. The disease breaks out unpredictably in Africa and tropical Latin America. The severity of the illness is variable.

Three to six days after the infective mosquito bite, victims experience muscle aches, fever, headache, loss of appetite, nausea and vomiting. At this stage it’s hard to differentiate from flu and myriad other causes of fever in travellers.

Often victims will start to recover three to four days after the start of symptoms; however, up to a quarter relapse into a very serious phase, when the blood’s ability to clot is compromised (people bleed from gums and internally) and jaundice starts. Hospital treatment is ‘supportive’, which means there is no cure but sometimes intervention in intensive care will save a life. Up to half of patients who develop the serious form of the disease die.

Why the fuss about the certificate?

A yellow fever International Certificate of Vaccination or Prophylaxis (ICVP) is required for travel to some countries – but rules are nothing to do with protecting individual travellers from catching the disease. Yellow fever is spread from infected people or monkeys to other victims by way of Aedes mosquito bites.

Aedes are widespread: they are found all over Asia, and in the warmer regions of North America and Europe, which means there is a risk that yellow fever could expand beyond its current range. Indeed, it was originally an African infection that spread to the Americas, probably via Aedes hitching lifts aboard slave ships.

The purpose of the ICVP is to protect the host country from visitors importing the disease and seeding it there. This means that someone travelling from Europe to a yellow fever risk-zone may not be required to prove that they have been immunised, whereas people travelling between countries with yellow fever will need to show a certificate.

Note that if you’ve visited a risk destination in the weeks just before entering another country, you will need a certificate. For instance, Kenya doesn’t require travellers coming from the UK to hold a certificate, but if you’d recently returned from a trip to Uganda, then you would need an ICVP.

What is crucial is that people understand the difference between their individual risk and the local immigration requirements. Just because your destination does not require you to have an ICVP in order to enter the country, that doesn’t mean that there is no yellow fever risk there.

Is the vaccine effective?

Yes, the vaccine is fully protective. Also, although it officially protects for ten years, evidence is emerging that one single dose gives lifelong immunity. However, international regulations are somewhat lagging behind scientific understanding and it’s hard to know when the rules will change at borders; currently there is still a requirement to show that immunisation has been administered some time between ten days and ten years previously.

Are there side effects?

Side effects are common. Between 10% and 30% of people receiving the vaccine report mild fever, arm soreness, headache or aches. There have been reports of severe reactions, but this is rare – estimated at less than five cases per million doses.

The chance of a severe reaction is five times greater in people over the age of 60; even so, the risk is low, at one per 40,000 doses. This may need to be considered when older travellers who’ve never received the vaccine plan a trip to a yellow fever risk area.

Vaccination waivers can be issued on medical grounds by travel clinics – although, obviously, having a waiver will not protect the traveller from contracting yellow fever. Bite avoidance is key. Aedes, which bite from dawn to dusk, are repelled by DEET and EX-4, though note they can bite through thin shirts and will bite any exposed skin – not just ankles, like malarial mosquitoes.

Is the vaccine safe for everyone?

No. Children under five months must not receive yellow fever vaccine, nor must people who are immunosuppressed (eg cancer patients) and those who take steroid tablets in high doses (more than 40mg prednisolone).

It is also unsuitable for those with an egg allergy. People who may not be able to receive yellow fever vaccine include children under nine months, pregnant and breastfeeding women, and people with HIV and low CD4 counts.

What is forced immunisation?

If there is a yellow fever outbreak, occasionally public health workers will immunise around it, effectively trying to make a fire break. This means that buses in the vicinity could be stopped and all passengers given the vaccine; it also means border officials might insist on giving the immunisation. Such situations can be alarming as it is not possible to be sure how sterile the needles are. Holding a vaccination certificate should prevent forced immunisation; a waiver may not.


Dr Jane Wilson-Howarth has contributed health features to Wanderlust since the first issue in 1993, and is author of The Essential Guide to Travel Health (Cadogan, 2009). Browse her article archive and bio here.