Another day, another measles outbreak - this time more than 100 children have been affected by measles in an outbreak traced back to Disneyland. Once again, the cause is thought to be inadequate levels of immunisation against measles.
Doctors recommend immunisations for two main reasons. The first is to protect the person being vaccinated from getting the disease. All vaccines are designed to prompt your immune system to react - the white cells in your body recognise the vaccine and fight it off, which is how they become primed to pounce on the 'real thing' if they encounter it.
The second reason for immunisation is to boost 'herd immunity', for everyone in the population. If enough people are immunised against a disease, it can't find enough hosts to spread easily. That slows down the rate of spread through the community, and if the 'pool' of non-immune people is small enough, it can stop an outbreak before it ever gets going. For diseases like measles and mumps, herd immunity happens when at least 90% of children are immunised.
Vaccines are either made from a weakened version of the original virus, which can't spread and doesn't cause as many side effects as the real thing, or from tiny particles of the virus which aren't alive. So after MMR immunisation, like any other immunisation, many children will develop a mild fever and rash seven to ten days after the immunisation, as the immune system kicks into action.
In 1968, a combined immunisation against measles, mumps and German measles, or rubella, was introduced into the UK immunisation schedule. Until then, there were about half a million cases a year in the UK. While most recovered, many developed complications, with about 100 of them dying, up to one in 20 diagnosed with pneumonia, one in 200 suffering seizures and up to one in 1,000 getting encephalitis with a risk of brain damage. After the vaccine was introduced, numbers of people affected plummeted, and doctors began to believe that like polio, we might be able to consign the suffering caused by these diseases to distant memory.
There were many reasons for introducing a triple vaccine, rather than single ones. The triple vaccine was much more extensively researched, so its data was more robust (after the MMR scare in the UK, many parents who paid for separate immunisations for their children had be recalled because it turned out the single immunisations their children had received weren't effective). Giving one immunisation rather than three at different times dramatically improves uptake - so increasing the chance of herd immunity.
Every parent wants to do the best for their child, and it's almost inevitable that they will be concerned by the smallest hint of doubt over safety for something they're asked to give their child for their protection. And in 1998 one man, Andrew Wakefield (previously a doctor in England but struck off the British Medical Register in 2010 by the General Medical Council after being found guilty of serious professional misconduct over the research which led to his claims) made headlines across the world by publishing a paper claiming a link between the MMR vaccine and autism.
There's no doubt that the number of children diagnosed with autism has increased in recent years. Doctors and scientists are desperate to find a cause, and one of the best ways to do this is to see what happened to the rate of rise before and after MMR was introduced. If there was a sudden surge in cases in the few years after MMR was widely taken up, this would suggest strongly that there might be a link between the two.
By contrast, if the number of children continued to rise at exactly the same rate when MMR was introduced, and over several years when uptake levels were stable, this would indicate equally strongly that something other than MMR was to blame. The best study from a scientist's perspective is a 'prospective' study - one that starts collecting data when the study starts, rather than looking backwards. But large studies involving many thousands of subjects are usually reliable if they're looking forwards or backwards. Many such studies were done (refs 1-5), looking over periods of six to 14 years and some including up to 1.8 million children (compared to Wakefield's initial study, which involved just 12 children). They all reached the same conclusion, summed by a group of UK researchers (5):
"Because the incidence of autism among 2 to 5 year olds increased markedly among boys born in each year separately from 1988 to 1993 while MMR vaccine coverage was over 95% for successive annual birth cohorts, the data provide evidence that no correlation exists between the prevalence of MMR vaccination and the rapid increase in the risk of autism over time. The explanation for the marked increase in risk of the diagnosis of autism in the past decade remains uncertain."
I am a mother as well as a doctor, so I understand parents' concerns about immunisations. But as a doctor, I was in a position to let the facts get in the way of a good headline. I knew that while most children recover from these diseases, about one in 10 ends up in hospital and many suffer lasting complications or even die. My daughter became due for her immunisation when the scare story was at its height. Did I get her immunised after I had read all the evidence? Without hesitation.
1) Peltola H, Patja A, Leinikki P, Valle M, Davidkin I, Paunio M. No evidence for measles, mumps and rubella vaccine associated inflammatory bowel disease or autism in a 14year prospective study. Lancet 1998;351:13278.
2) Taylor BME, Farrington CP, Petropoulos MC,
FavotMayaud I, Li J,Waight PA. Autism and measles, mumps and rubella vaccine: no epidemiological evidence for a causal association. Lancet 1999;353:20269.
3) Jatja A, Davidkin I,Kurki T, Kallio MJT,Valle M, Peltola H. Serious adverse events after measles/mumps/rubella vaccination during a fourteen year prospective follow up. Pediatr Infect Dis J 2000;19:112734.
4) BMJ 2001;322:183-4
5) Kaye J, del Mar MeleroMontes M, Hershel J. Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend analysis
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