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Children's Vaccines Health Center

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Study: Vaccines Don't Cause Autism

MMR, Thimerosal-Exposure Not Linked to Incidence

Unique Opportunity continued...

"Our study once again rules out MMR as a cause for autism," Fombonne tells WebMD. "Because both doses were given prior to age 2, this was a unique opportunity to study this vaccine at the time in which these disorders are usually first seen."

Fombonne and colleagues also had a unique opportunity to study the impact of thimerosal dosage, because children born at different times during the study period had very different exposures.

Those born between 1987 and 1991 had what Fombonne called medium cumulative exposures to thimerosal, while those born between 1992 and 1995 had higher levels because the Hib vaccine was added to the immunization schedule. Hib vaccine protects against a type of bacteria which can cause serious infections in children (such as meningitismeningitis, pneumoniapneumonia, and infections of the bones, blood, and joints). Thimerosal was removed from vaccines given to Canadian children in 1996, so children immunized after this time had no exposure.

Despite the changes in exposure, the incidence of autism and autism-related disorders continued to increase in a linear manner during the study period, leading the researchers to conclude that thimerosal exposure did not affect autism rates.

The Search for Answers

So if vaccines are not contributing to the rise in reported autismautism cases, what is? Fombonne says the increase can be explained by a broadening of what is considered autism and related disorders, increased awareness of these disorders, and a greater emphasis on early diagnosis.

American Academy of Pediatrics spokesman Joseph Bocchini, MD, tells WebMD that while it is clear that these are important contributors to the increase in diagnosed cases, other unidentified contributors may also be playing a role.

Bocchini is chief of pediatric infectious diseases at Louisiana State University Health Sciences Center in Shreveport.

"We don't know if there is an actual increase in incidence, but we are working to find that out," he says.

He adds that the Canadian study supports a "large body of data" showing no association between MMR vaccines or thimerosal exposure and pervasive developmental disorders like autism.

"It is understandable that parents of children with these disorders would want to find something that they can point to as a cause, and it is also understandable that they would look at vaccines. But when you look at the evidence in an objective way there is simply nothing to link vaccines with these disorders."

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