By Amy Norton
FRIDAY, March 29 (HealthDay News) -- Although some parents worry about the sheer number of vaccines babies typically receive, a new U.S. government study finds no evidence that more vaccinations increase the risk of autism.
Looking at about 1,000 U.S. children with or without autism, researchers at the Centers for Disease Control and Prevention found no connection between early childhood vaccinations and autism risk.
Children with autism and those without had the same total exposure to vaccine antigens -- the substances in vaccines that trigger the immune system to develop infection-fighting antibodies.
"This should give more reassurance to parents," said lead researcher Dr. Frank DeStefano, director of the CDC's Immunization Safety Office.
The findings, which appear online March 29 in the Journal of Pediatrics, cast further doubt on a link between vaccines and autism spectrum disorders -- a group of developmental brain disorders that impair a child's ability to communicate and socialize.
The first worries came from a small British study in 1998 that proposed a connection between the measles-mumps-rubella (MMR) vaccine and autism. A spate of research since has found no link, and the original study was eventually retracted by the Lancet, the journal that published it.
Then came concerns about thimerosal, a preservative once used in certain childhood vaccines (but never MMR) that contains small amounts of ethyl mercury. Again, international studies failed to show a link to autism.
More recently, worries have shifted to the notion that children are getting "too many vaccinations, too soon." In the United States, children can be immunized against 14 different diseases by the time they are 2.
DeStefano said his team focused on antigen exposure, rather than just the number of vaccinations, because that gives a more precise idea of the "immune system stimulation" kids received through vaccines.
A recent survey found that about one-third of parents thought children receive too many vaccinations in their first two years of life, and that the shots could contribute to autism.
But there's no scientific evidence of that, said Dr. Paul Offit, chief of infectious diseases at the Children's Hospital of Philadelphia.
He said it's understandable that parents might worry. "You see your baby receiving all these vaccines. It looks like too much. It feels like too much," Offit said.
But, he said, there's no biological basis for the idea that vaccines "overstimulate" the immune system, and that somehow leads to autism.
Every day, babies' immune systems battle many more antigens than are present in vaccines, DeStefano explained. "Most infants can handle exposure to many antigens," he said.
The CDC team found that kids' total antigen exposure in the first two years of life was unrelated to their risk of developing an autism disorder.
That was also true when they considered babies' antigen exposure in the first three months of life, and the first seven months. Nor was there any connection between autism risk and the amount of vaccine antigens children received on any single day.
"This provides evidence that concerns about immune system overstimulation are unfounded," DeStefano said.
Geraldine Dawson, chief science officer for the advocacy group Autism Speaks, said the study "adds to the existing literature showing no connection between vaccines and autism in large epidemiological studies."
She added, though, that further research is needed "to explore whether, in rare cases, a genetic vulnerability might increase susceptibility to vaccine-related side effects, including the triggering of autism symptoms in a genetically and medically susceptible child."
Both Offit and DeStefano stressed that there is no reason for parents to delay vaccinating their child.
"This is one more piece of evidence to help reassure parents," Offit said.