BBC Future

The Real Risk

Vaccines: Risks and benefits

About the author

David Spiegelhalter is Winton Professor of the Public Understanding of Risk at the University of Cambridge, UK. His professional background is in medical statistics. He is a frequent contributor to the media, and has been an advisor on risk issues as diverse as breast implants, volcanic ash, clinical trials, surgical mortality, and the inquiry into mass murderer Harold Shipman. He is a Fellow of the Royal Society and was awarded an OBE in 2006. You can see more of David’s explorations of risk at the Understanding Uncertainty website, or follow him on Twitter at @undunc

Vaccines

(Copyright: Thinkstock)

As high-profile cases have shown, causes, coincidences and effects mean that balancing risks and benefits is not always a straightforward task.

Pity the small boy. When I was a lad, there were no vaccinations against measles, mumps and chicken pox, so when someone local had a dose of a disease I was marched round to be infected. I realise now that measles exposed me to around a 1 in 500 chance of death (see later), but there wasn’t much choice back then, and no doubt it was character forming.

Nowadays, of course, we have vaccines to do the job of small children. But as successful as campaigns have been in saving countless lives, some have aroused strong emotions, as a result of ticking several fear-factors. First, we inject healthy people, usually vulnerable children, and it’s imposed, either through pressure or by legal compulsion. If your child is to attend a kindergarten in, for example, Florida, they must have been vaccinated against the following: DTaP – diphtheria, tetanus, pertussis (whooping cough); Hepatitis B; MMR – measles, mumps, rubella (German measles); polio and varicella (chicken pox). Added to this is the fact that there can be side-effects. And finally, multinational corporations make a heap of money out of this mass medicalisation.

All of which is true. Little wonder, then, that claims that vaccination may cause adverse outcomes such as autism find a ready audience.

Health check

But we can work out roughly the risks without immunisation by tracking the course of a disease like measles over the decades. In England and Wales in 1940, just over a decade before I was born, there were 409,000 measles cases, of which 857 died – a ‘case fatality rate’ of 0.2%, which is also that quoted by the Centers for Disease Control and Prevention (CDC) in the US. In other words, the 1 in 500 chance of death I mentioned earlier. Vaccination started in the 1960s, and by 1990 the number of cases had dropped to 13,300 with one fatality. Since 1992, there have been no childhood deaths from measles in the UK, only as adult consequences from early infections. 

So it seems rather a good thing to be vaccinated and, rather like stopping smoking, it is also good for the people around you. This is because of herd immunity, which means that sufficient people are immune so that an infection does not turn into an epidemic. The current English vaccination rates for measles (as of 2009) are 88%, up from 80% in 2003 – but still not back to the 92% level in 1995, let alone the 95% recommended by the World Health Organization (WHO). In 2010, US vaccination coverage for children between 19 and 35 months of age was below 90% in eleven states.

Measles is the first M in the MMR vaccination, and coverage went down after the highly publicised claim in 1998 that MMR was associated with autism. This has now been discredited, although it continues to have strong supporters in the US – just try searching on “vaccine autism”. And its impact continues to be felt far and wide. After an outbreak of measles in Liverpool in February 2012, the UK’s Health Protection Agency revealed that 7,000 children under five years of age had not received their full measles vaccine.

Risk assessment

The real problem is that with any mass intervention there will always be bad occurrences that happen around the time of the jab – essentially coincidences. For example, in September 2009 a headline in the UK newspaper Daily Mail declared that “Schoolgirl 14 dies after cervical cancer jab”, quoting the head teacher as saying, “During the session an unfortunate incident occurred and one of the girls suffered a rare, but extreme reaction to the vaccine.” Three days later reports revealed that the girl had cancer and the death was coincidental: however this was not headline news, and this tragic event is used repeatedly on websites as proof of the dangers of the HPV vaccine.

But sometimes the reports are real. A classic example occurred in 1976 when a new strain of swine flu was identified in Fort Dix, New Jersey.  Fearful of a repeat of the 1918 epidemic, a mass vaccination campaign began, and 45 million people were immunised.

Two events led to the abandonment of the programme by the end of the year. First, there were around 50 reported cases of Guillain-Barré syndrome - a gradual paralysis that is now thought to have been former US president Franklin D Roosevelt’s condition. Eventually 500 cases were reported among vaccinated people – an increased risk of around 10 in a million for the disease – and 25 people died.  The second reason for stopping the programme was that the epidemic never got out of Fort Dix – nobody else had the flu and so there seemed no upside to balance out the possible risk of Guillain-Barré syndrome. The Director of the CDC was later sacked, but he still believes the vaccination programme was the correct response. 

That said, not all flu vaccines have the same risks. Following the UK swine flu outbreak in 2009, nine cases of Guillain-Barré syndrome were diagnosed within six weeks of vaccination; however, the eventual conclusion was that this would be expected by chance alone. But Finland and Sweden have reported increased rates of narcolepsy – sudden paralysis and sleepiness – in children after the swine flu vaccination, and this is still being investigated.

Balancing risk

As the MMR saga showed, disproving an association is difficult and can take a long time, if indeed ever. Sometimes a change is made even without absolute proof of guilt. Thimerosal is a preservative used in some vaccines and contains mercury, and has long been accused of harming children. The CDC say there is “no convincing evidence of harm”, but in 1999 it was agreed that it should be “reduced or eliminated in vaccines as a precautionary measure”. 

The official line that the overall benefits of vaccination outweigh any risks ignores the way in which imposed and highly visible harms, however rare, are seen very differently from potential downstream benefits, which can never be confirmed and seem ‘virtual’ in societies where the risks of infectious diseases are so low. 

It is a different matter in less-developed societies: for instance, the WHO report that there are still 140,000 deaths from measles each year, one every four minutes. And, as we have seen in England, these are preventable. Vaccination has already made huge inroads: there used to be 2.6 million deaths a year from measles worldwide. Eradicating measles is seen to be a feasible goal, and the days of being dragged round to someone’s house to get infected are thankfully over. But as the numbers show, whatever the potential risks of vaccinations are, they pale in comparison to the risk in shunning what is often our best option for eradicating deadly infectious diseases. 

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