Dear ATC Readers,

Recent news reports have raised concerns about yellow fever vaccine safety. It is important to understand the risks and benefits.

For decades, yellow fever vaccine was considered to be extremely safe. More recently, however, we have become aware of a low rate of serious adverse effects occurring when older travellers are vaccinated for the first time. The two main types of adverse effects are called YEL-AVD and YEL-AND – the first involves damage to organs such as the liver, and the second damages the nervous system. Yellow fever vaccines contain live viruses that do not harm people with a normally functioning immune system, but do seem to cause harm in a tiny proportion of people with an aging immune system, as well as those with reduced immunity from other causes.

In one of the largest medical studies of yellow fever vaccine safety, the rate of YEL-AND in people aged 60-69 was 2.5 per 100,000 people vaccinated, and in over 70s was 1.6 per 100,000. For YEL-AND, the rates were zero and 4.0 per 100,000 respectively.

According the US Centers for Disease Control, in the absence of a yellow fever outbreak, the “background” risk of yellow fever occurring in unvaccinated travellers on a 2-week trip to a zone where yellow fever is present is calculated to be:

• West Africa: 50 cases per 100,000, with 10 deaths per 100,000
• South America: 5 cases per 100,000, with 1 death per 100,000

Yellow fever vaccinating centres normally explain these risks to everyone they vaccinate, and in particular to travellers aged over 60.  Vaccination for personal protection is recommended where benefit outweighs the risks, and a medical waiver is given when it is not safe for the vaccine to be used. Age alone is not a reason for issuing a waiver.

Other factors that complicate the picture include the following:

• There are active outbreaks of yellow fever in several parts of the world, notably at present in Brazil as well as in several African countries, so the risk to travellers is often much higher than that quoted above.
• Vaccination is the only way to prevent and control yellow fever, and is often to only possible public health measure for entire populations.
• Many countries therefore rigorously enforce vaccine requirements as a condition of entry.
• Yellow fever vaccine shortages are common – there is currently a serious shortage in the USA, and vaccine is not always available when needed.
• Awareness of vaccine risks has come with a stricter approach to recommending vaccine only when travellers are definitely going to be at risk; however, an unintended consequence of this has been to delay the opportunity to vaccinate travellers for the first time until they are older and at greater risk of adverse effects.

The World Health Organization now considers vaccine protection to be life-long, though border officials in several countries do not always recognise this. A small number of recent cases of yellow fever in Brazil among people who had been vaccinated previously has raised further doubt about this policy, and Brazil currently recommends revaccination every 10 years.  Revaccination has not been linked to adverse effects.

There is currently extensive UK press coverage (that has also been picked up in the US) of YF vaccine adverse effects – one death shortly after vaccination in a distinguished cancer researcher, and a psychotic reaction in a former BBC journalist vaccinated in Greece. Both cases were attributed to the vaccine because of the timing. Further clinical details are not available, so we can’t yet be certain of a direct link, but this news will undoubtedly have a considerable impact on public perception of the vaccine.

Dr Richard Dawood

Fleet Street Clinic
29 Fleet Street

London EC4Y 1AA