July 6, 2006 -- New research from Canada may not end the debate about childhood vaccines and autismautism, but it offers more evidence that vaccines are not to blame for the dramatic rise in reported cases of the developmental disorder.
The study examined outcomes among 28,000 children in Quebec, exposed to different dosages of the measlesmeasles, mumpsmumps, rubella (MMR) vaccine and vaccines containing the mercury-based preservative thimerosal. Researchers found no relationship between MMR vaccine exposure, thimerosal exposure and autism rates.
In fact, a higher incidence of autism was seen in Canadian children vaccinated after thimerosal was eliminated from vaccines than among children who received thimerosal-containing immunizations.
The study is published in the July issue of the journal Pediatrics.
"We found that the prevalence of autism and other pervasive developmental disorders was higher among kids who had zero exposure to mercury than among kids with what would be considered medium and even high exposures," says researcher Eric Fombonne, MD, who directs the department of pediatric psychiatry at The Montreal Children's Hospital.
Fombonne is a long-time autism researcher who documented the rise in cases in the U.K. before joining the faculty of Canada's McGill University.
His latest study shows that autism rates increased steadily in Canada among children born in the study period between 1987 and 1998. The increase was similar to that which has been reported in the U.S. and the U.K., he tells WebMD.
Canadian health officials added a second MMR vaccine prior to age 2 in children vaccinated after 1995, so children immunized prior to this received half the dosage of the vaccine as children vaccinated later.
MMR vaccines have never contained mercury, but it has still been believed by some to be a possible cause of autism. After reviewing the clinical evidence, a panel convened by the independent Institute of Medicine rejected this idea in its 2004 report on vaccine safety. The panel also found no link between thimerosal and autism.
The increase in autism or autism-related disorders seen among the children in the study remained steady before and after 1996, suggesting that increasing the MMR dosage had no impact on the developmental disorder.
"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."