Study: Vaccines Don't Cause Autism
MMR, Thimerosal-Exposure Not Linked to Incidence
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.