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By Amy Norton
NEW YORK, Mar 06 (Reuters Health) - Despite a possible link between childhood MMR vaccination and autism found in one well-publicized study in the UK, there is growing evidence that no such connection exists. The latest comes from an analysis of California statistics showing that while autism cases in the state grew rapidly during the 1980s and early 1990s, the increase showed relation to vaccination rates.
In the debate over vaccination and autism, these findings may be particularly important because some reports have cited California's jump in autism cases as evidence that the disorder may be caused by the measles-mumps-rubella (MMR) vaccine. The MMR was introduced in the US in the 1970s and is routinely given to babies older than one year.
Dr. Loring Dales and his colleagues at the California Department of Health Services in Berkeley compared the number of autism cases reported in the state from 1980 to 1994 with rates of MMR vaccination during the same period. The investigators found that autism cases exploded by 373%, but the percentage of children who got the MMR by age 2 increased only moderately, from 72% to 82% over the 14-year period.
The researchers also discovered that the jump in autism cases started before a small 1988 increase in rates of MMR vaccination, and that the autism increase steadily continued after MMR rates had leveled off.
These disparate patterns show that MMR vaccination could not have triggered California's steep increase in autism, Dales told Reuters Health in an interview.
"We can say, for certain, that theory is incorrect," he said.
Dales and his colleagues report their findings in the March 7th issue of The Journal of the American Medical Association.
In 1998, UK researchers reported that they had found a possible link between the MMR vaccine and autism in 12 patients with autism and bowel disease. The UK team speculated that the vaccine had caused the bowel problems, which in turn led to poor nutrient absorption and impaired brain development.
But autism is a complex developmental disorder, and researchers believe that genetic and environmental factors conspire to trigger it. According to the National Institute of Child Health and Human Development, there is no good evidence that bowel disease and poor nutrient absorption can cause autism.
Since the MMR-autism theory was put forth, studies in the UK, Sweden and the US have shown that patterns of vaccination and autism among children do not match. In the case of California, Dales explained, people have looked at the escalating number of autism cases since the 1980s and linked it to the standard use of the MMR vaccine around that time. But, he said, they did not investigate the actual rates of MMR use during the 1980s and 1990s.
"There is no correlation to show that (MMR vaccination) is a major factor, or even a factor at all, in autism," Dales said.
However, he noted, it remains unclear whether the vaccine may be behind a "tiny portion" of autism cases.
As for the stark increase in California's autism cases, Dales said, experts debate whether it is a "true increase" or the result of wider recognition and better diagnosis of the disorder.
SOURCE: The Journal of the American Medical Association 2001;285:1183-1185.
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