Study: Autism Prevalence Still Up After Thimerosal Removed From Vaccines

A study in the January 2008 Archives of General Psychiatry does not support the hypothesis that childhood exposure to thimerosal-containing vaccines, or TCVs, is linked to autism.

Thimerosal is a preservative that contains ethylmercury. Vaccines were the main source of thimerosal exposure among young children in the United States before the preservative's removal from vaccines in recent years.

The authors of "Continuing Increases in Autism Reported to California's Developmental Services System" are Robert Schechter, M.D., M.Sc., and Judith Grether, Ph.D., both with the California Department of Public Health in Richmond. The department supported the study.

After analyzing autism client data from the California Department of Developmental Services, or DDS, the researchers concluded that the data "do not show any recent decrease in autism in California despite the exclusion of more than trace levels of thimerosal from nearly all childhood vaccines."

The researchers found that the prevalence of autism reported to the DDS actually has increased consistently among children born from 1989 through 2003, which includes the period when exposure to TCVs decreased.

"Moreover, since 2004, the absolute increase and the rate of increase in DDS clients aged 3 to 5 years with autism were higher than those in DDS clients of the same ages with any eligible condition, including autism," the authors write. "These time trends are inconsistent with the hypothesis that thimerosal exposure is a primary cause of autism in California."

The new study is significant because it relies on the same database that has been "systematically used by proponents of the thimerosal hypothesis," says a commentary in the same issue by autism expert Eric Fombonne, M.D., director of the department of psychiatry at Montreal Children's Hospital of the McGill University Health Centre. The data analyzed in the new study "provide a clear and unambiguous test that shows that the expected decline in autism rates following discontinuation of thimerosal in U.S. vaccines did not occur," said Fombonne.

Despite the accumulation of evidence refuting the thimerosal connection to autism and despite evidence rejecting another theory linking autism to the measles component in the measles-mumps-rubella vaccine, these theories and practices related to them haven't faded away, Fombonne writes. "How many more negative study results are required for the belief to go away, and how much more spending of public funds on this issue could even be justified?"

"Parents of autistic children should be reassured that autism in their child did not occur through immunizations," Fombonne concludes. "Their autistic children, and their siblings, should be normally vaccinated, and as there is no evidence of mercury poisoning in autism, they should avoid ineffective and dangerous 'treatments' such as chelation therapy for their children."

In other autism news, scientists with the David Geffen School of Medicine at the University of California, Los Angeles, have identified a new gene linked to autism. They report their findings in the Jan. 10 online edition of the American Journal of Human Genetics. That journal issue also includes studies from researchers at Yale University School of Medicine in New Haven, Conn., and Johns Hopkins University School of Medicine in Baltimore, who used different methods to identify the same gene.

According to a Jan. 10 news release, the UCLA researchers found that the gene, called contactin-associated protein-like 2, or CNTNAP2, is most active in brain regions involving language and thought. "The fact that we found CNTNAP2 concentrated in the brain's structures that are involved in higher cognition gives us strong clues about how its disruption might adversely shape brain development, including speech and language," says Brett Abrahams, M.D., Ph.D., one of the researchers, in the release.

Somewhat unexpectedly, the researchers also determined that statistical evidence for the gene was strongest in families with autistic boys compared with families with autistic girls or those with autistic girls and boys.

"Autism strikes boys three times as often as girls," says Maricela Alarcon, Ph.D., another of the study's researchers, in the news release. "This finding may partly explain why."