Although some research has supported a link between exposure to the vaccine preservative thimerosal and autism, a new analysis of data from the California Department of Developmental Services (DDS) indicates that removal of this chemical from vaccines has not reduced autism rates. In fact, the rates have actually continued to rise.
"The DDS data do not support the hypothesis that exposure to thimerosal during childhood is a primary cause of autism," Dr Robert Schecter and Dr Judith K. Grether, from the California Department of Public Health in Richmond, write in the Archives of General Psychiatry.
After thimerosal was linked to autism, removal of the chemical from vaccines, which had already begun, was accelerated in the United States from 1999 to 2001, according to the report. In the current study, the researchers analysed DDS data to examine if autism rates changed from 1995 through March 2007.
Prevalence still rising
The overall prevalence of autism for children between 3 and 12 years of age rose during the study period. For example, among 3-year-olds born in 1993, there were 0.3 cases per 1 000 births. The corresponding rate for children born in 2003 was 1.3 cases per 1 000 births.
The highest prevalence of autism - 4.5 cases per 1 000 births - was reported in 2006 among children born in 2000.
"These time trends are inconsistent with the hypothesis that thimerosal exposure is a primary cause of autism in California," the researchers point out.
However, they also support the continued search for modifiable risk factors for the condition, along with continued surveillance of the prevalence of autism to confirm the current findings.
The current findings "provide additional evidence of the lack of association between thimerosal exposure and the risk of autism in the US population," Dr Eric Fombonne, from the Montreal Children's Hospital, comments in a related editorial. The results are consistent with studies previously published and particularly with similar studies conducted in Denmark and Canada.
The study by Schechter and Grether was funded by the California Department of Public Health.
SOURCE: Archives of General Psychiatry, January 2008. – (Reuters Health)
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