With significant public concern about the safety of the MMR vaccine, this week's column examines the scientific evidence for and against an association between MMR, autism and inflammatory bowel disease.
Reports of a link between MMR and autism began when Dr Andrew Wakefield and colleagues from the Royal Free Hospital in London published a paper in the Lancet in 1998, describing 12 children with developmental and bowel problems. Eight of the children had autism, which their parents reported began soon after their vaccination with MMR. The authors proposed that MMR caused a leaky bowel, which allowed a toxin to enter the body, travel to the brain and cause autism. However, a quote from the Wakefield study is significant. "We did not prove an association between MMR vaccine and the syndrome described," it stated.
The research was open to criticism on a number of fronts: the number of patients was small; there was no control group for comparison, and as it was carried out retrospectively, some parents were surveyed up to eight years after their children's vaccination, leaving the study open to the possibility of faulty recall.
Notwithstanding these scientific loopholes, the research caught the public's attention. Nor was it ignored by doctors and scientists, who set about designing other studies to test for a link between MMR and autism.
In 1999, Prof Brent Taylor, in London, published a paper in the Lancet, looking at the immunisation records of 498 children with autism who were born before and after the introduction of MMR. The researchers found no difference in the incidence of autism between the two groups.
A British Medical Journal paper in February 2001 found a notable rise in the diagnosis of autism by UK general practitioners from 1988 to 1993. However, over the same timespan, there was no change in the uptake of MMR vaccine, which remained constant at 97 per cent.
Across the Atlantic, a paper in the Journal of the American Medical Association compared trends over time between autism and MMR immunisation rates in California. It concluded that "these data do not suggest an association between MMR immunisation among young children and an increase in autism occurrence".
Prof Taylor has recently published what many regard as the definitive study into a possible link between MMR and autism. He and his colleagues looked at 473 children with autistic disorder in north-east London and linked them to an independent immunisation registry.
They found that, at two years of age, the MMR vaccination coverage among children with autism was almost identical to that of all children in the area. No temporal link could be found between MMR immunisation and the onset of autism.
Significantly, they found no sharp increase in the number of children with autism following the introduction of MMR in 1988. Children with autistic disorder immunised with MMR before 18 months of age, immunised after 18 months or never immunised at all had similar ages at diagnosis. This is strong evidence that vaccination with MMR did not result in the early onset of autism.
AN ALLEGED link between the measles virus and inflammatory bowel disease (Crohn's disease and ulcerative colitis) was first suggested in 1993. Again it was Dr Wakefield who initially claimed that the virus was present in the gut of patients with Crohn's disease, although when he repeated his original work using more sensitive tests, no measles virus was identified.
A British study of more than 7,600 people compared those who had received live measles vaccine in childhood with those who did not. It found no difference for the risk of ulcerative colitis or Crohn's disease between the two groups. A large Finnish study of 1.8 million people vaccinated with MMR, who were followed up for 14 years, found no link with inflammatory bowel disease.
While there is no doubt that the incidence of autism has increased in recent years, there is no scientific evidence to link the finding with the MMR vaccine. Nor has a scientific link been established with Crohn's disease. To quote the authors of a review in the most recent volume of Clinical Evidence, a respected source for the best available healthcare evidence: "We found that the \ study does not establish MMR as a cause of inflammatory bowel disease, autism or developmental regression and that its hypothesis has been satisfactorily tested by other scientifically reliable studies."
E-mail Dr Muiris Houston, Medical Correspondent, at mhouston@irish-times.ie or leave
a message at 01-6707711 ext 8511. He regrets he cannot reply to individual medical problems