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August 14, 2008

California Data Do Not Support a Link Between Thimerosal in Vaccines and Autism

Posted 08/07/2008
William T. Basco, Jr., MD, FAAPAuthor Information
Information from Industry
Continuing Increases in Autism Reported to California's Developmental Services System: Mercury in Retrograde
Schechter R, Grether JKArch Gen Psychiatry. 2008;65:19-24
This study was a retrospective review of autism spectrum disorder (ASD) cases referred to the California Department of Developmental Services (DDS) from 1995 through 2007. Children enter the DDS system at 3 years old; therefore, this referral represents the prevalence of a diagnosis of ASD in this population at the age of 3 years.
The authors plotted the prevalence of ASD referrals against critical time points in US vaccine history relative to the use of thimerosal. Critical dates relative to thimerosal are:
1991: The addition of vaccines to the recommended schedule produces an increase in the total mercury exposure (via thimerosal) in vaccines;
1999: A recommendation to remove thimerosal from vaccines was made;
2001: Thimerosal is no longer present in vaccines except in trace quantities;
2002: The overall expiration time period of vaccines with anything more than trace amounts of thimerosal is reached.
The authors' hypothesis was that if thimerosal were a contributor to ASD then children born and vaccinated after 2002 should have decreased rates of ASD diagnosis, and this would be reflected in decreased numbers of referrals (ie, decreased prevalence) to California DDS.
The authors did not find a dip in referral rates; their data did not reflect a change in ASD prevalence. In fact, the prevalence of ASD was increasing before 1991 and continued on roughly the same slope throughout the 1990s and 2000s, regardless of age group evaluated.
Only one year's cohort of children was born after thimerosal was largely eliminated from vaccines and were not at risk for receiving old vaccine -- the children born in 2003. These children also had a higher prevalence of ASD than children born in previous years, and the slope was increasing along a very similar trajectory to prior years. The increase over the time period 1993 through 2003 was from 0.3 cases per 1000 births to 1.3 cases per 1000 births.
The authors conclude that the California DDS data do not support a link between thimerosal in childhood vaccines and development of ASD.
This study was published in January 2008 but only recently came to my attention. The findings here mirror those of a Danish study that demonstrated ongoing increases in ASD diagnosis despite removal of thimerosal from vaccines,[1] as well as Canadian data demonstrating no decrease in ASD diagnosis for children vaccinated with non-thimerosal-containing vaccines.[2] The figure from the Canadian study is particularly interesting and is worth viewing separately.

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