Research Focused on Thimerosal-Containing Vaccines
By Salynn Boyles
WebMD Health News
Latest Healthy Kids News
Reviewed by Laura J. Martin, MD
Sept. 13, 2010 -- Exposure to thimerosal-containing vaccines in infancy or in the womb is not associated with an increased risk for developing autism, according to a new study from the CDC.
Children in the study who developed autism spectrum disorder (ASD) actually had less exposure to vaccines with the mercury-containing preservative than children who developed normally.
The study is the latest of almost 20 studies to find no link between childhood vaccinations and autism.
It comes seven months after the first study that linked vaccines and autism -- conducted 12 years ago -- was retracted by the journal The Lancet. The U.K. doctor who published the study was banned from practicing medicine.
Cases of autism continue to rise throughout the world. The CDC now estimates that as many as one in 110 children in the U.S. develop ASD, which includes a range of developmental disorders from Asperger's syndrome to severe retardation and almost total social isolation.
CDC Director of Immunization Safety and study researcher Frank DeStefano, MD, MPH, tells WebMD that while the reason some children develop ASD remains a mystery, the focus should now shift to other potential causes.
“I don't think there is much worthwhile to study anymore with regard to thimerosal-containing vaccines and autism,” he says.
The CDC researchers examined records from three managed care organizations (MCOs) to identify 256 children with ASD born between 1994 and 1999 and 752 children without autism matched to cases by age, gender, and MCO.
Exposure to thimerosal-containing vaccines was determined using electronic immunization registries and medical charts. Interviews with parents were also conducted to confirm the autism diagnosis and vaccination history.
Researchers also recorded vaccines given to the children's mothers while they were pregnant.
Thimerosal was removed from most vaccines given to infants and children soon after the study participants were born. The one exception is most flu vaccines, which still contain the preservative.
The researchers found no increased risk for autism associated with prenatal exposure or exposure to thimerosal-containing immunizations in infancy or early childhood.
This included children who appeared to be developing normally through infancy into early childhood. About 20% of children with autism have this subtype of the disorder, known as ASD with regression.
The analysis indicated that children with the greatest exposures had slightly lower rates of autism than those who received fewer thimerosal-containing vaccines or none at all.
”This is a very nicely designed and carried out study that should reassure parents,” says pediatrician Margaret C. Fisher, MD, who is medical director of the Children's Hospital at Monmouth Medical Center in Long Branch, N.J.
Despite the overwhelming scientific evidence failing to support a link between childhood vaccination and autism, a recent study suggests one in four parents in the U.S. still believe vaccines might cause the developmental disorder.
In the online survey of parents with children and teens, 25% agreed, “some vaccines cause autism in healthy children.” Just over one in 10 parents said they'd refused an immunization for their children that a doctor had recommended.
Fisher tells WebMD she is not too surprised that so many parents still believe vaccines may cause autism despite the lack of scientific evidence to back up the belief.
She chairs the American Academy of Pediatrics' executive committee of the section of infectious diseases.
“I don't think we should expect that the science is going to completely counter what is a largely emotional response,” she says. “We are at a time in this country where there is a general distrust of science. I don't think people distrust their individual doctors, but there is distrust of the medical establishment.”
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