The prevalence rates of autism continue to rise, with the CDC now estimating that 1 in ever 150 children have autism, and researchers have made some progress in investigating possible causes of the disorder. Scientists have questioned whether repeated exposure to thimerosal, a preservative found in vaccines that contains ethylmercury, could contribute to the development of autism. Thimerosal was removed from all childhood vaccinations, except for some flu shots, in 2001. Now, a new study, published in this month's edition of Archives of General Psychiatry, reports that prevalence rates for autism in California increased consistently after the use of thimerosal in childhood vaccines was discontinued, an indication that there is no causal link between the preservative and autism.
Using data provided by the California Department of Developmental Services (DDS), the researchers measured rates of autism in children aged 3 through 12 who were referred to the DDS between January of 1995 and March of 2007. If thimerosal were in fact a major cause of autism, the number of children with autism referred for developmental services should had declined after 2001, when children's thimerosal exposure declined significantly. However, the researchers found that the number of children with autism in the DDS's database rose consistently from 1995 to 2007, and increases were seen in every year at every age level from 3 to 12 years of age. For instance, the researchers measured the rate of autism for 3 year olds to be 0.3 per every 1,000 live births during 1993. In 2003, when infants were no longer exposed to thimerosal in vaccines, the rate of autism had increased to 1.3 per 1,000 births. These results reinforce the conclusions of a 2004 review of all published and unpublished studies on the subject which found no evidence of a causal link between vaccines and autism.
There were some limitations to the study, especially concerning the nature of the data used. Because the DDS database was intended for administrative purposes and not for tracking the prevalence of developmental disorders, the study's results do not reflect the actual prevalence of autism in California. However, they point out that if thimerosal was a primary cause of autism, their data would still have showed a decline or a plateau following 2001.
The number of children diagnosed with autism in recent years has increased greatly, but many factors could be involved in this increase, including our improved ability to spot the disorder and a broadened definition of what exactly autism is. Future studies will continue to explore the potential genetic and environmental causes of autism. As we learn more about the ways in which the disorder develops and what changes we could make to reduce children's risk factors, we will be better able to treat, and perhaps prevent, autism.
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