Mar. 23 -- MONDAY, Jan. 7 (HealthDay News) -- Adding to a growing body of evidence that rejects the idea that immunizations boost autism rates, a new study finds no proof that incidences of the disorder dropped after makers of most childhood vaccines stopped using a mercury-based preservative in their products.
Researchers found that autism rates in California continued to rise over the past several years, even though the preservative -- known as thimerosal -- had vanished from almost all vaccines by 2001.
The study makes clear that "thimerosal cannot be the major cause of autism in California," said its lead author, Dr. Robert Schechter, medical officer with the Immunization Branch of the California Department of Public Health.
Another expert called the study limited and said it did not prove that vaccines have no connection to autism.
Still, the new research "adds to the body of existing evidence in which there is no causal connection that demonstrates thimerosal is a primary cause" of autism, said Andy Shih, vice president of scientific affairs for Autism Speaks, an advocacy organization.
Thimerosal, a preservative once used in contact lens solutions, was frequently a component of childhood vaccines until around 2000. Today, it's still used in flu vaccines recommended for infants, but researchers think children are still exposed to much less thimerosal than in the past.
In recent years, some parents have blamed their children's autism on the preservative, which is derived from mercury; others have accused the parents of creating public panic and threatening the health of children by casting a bad light on routine immunizations.
One government study released in 2007 claimed that thimerosal exposure in the first seven months of life didn't appear to affect the brain function of children aged 7 to 10, although there was some evidence -- perhaps the result of chance -- that connected thimerosal to later development of physical tics. Even earlier, an Institute of Medicine report released in 2004 found no evidence supporting a link between thimerosal and autism.
In the new study, published in the January issue of the Archives of General Psychiatry, researchers from the California Department of Public Health examined statistics about children in the state from 1995 through March 2007.
The prevalence of autism among kids aged 3 to 12 grew each year, the researchers discovered after looking at numbers compiled by a state agency that provides services to children with the disorder.
For example, the prevalence among kids born in 1993 was three in 10,000 when they were checked in 1996, compared to 13 per 10,000 among those born in 2003 and checked in 2006.
The highest rate -- 4.5 cases per 1,000 births -- came among 6-year-olds in 2006 who were born in 2000.
The study authors pointed out that autism rates among kids aged 3 to 5 continued to grow for each birth year after 1999, even though thimerosal use dropped.
Shih said the study methods are "robust," adding that the authors appeared to address limitations of earlier studies. However, he said, the research "doesn't address the possibility that there might be a subpopulation who might be particularly vulnerable to a vaccine with thimerosal."
If just 1 percent to 2 percent of children were especially sensitive to the effect of thimerosal, the study wouldn't be able to pick it up, he said.
Schechter agreed, saying it was true that the research didn't address whether thimerosal might cause problems in a small number of cases.
As for a possible link between vaccines and autism, Shih said, "the jury is still out," especially considering that children are exposed to a large number of vaccines before age 2.
And he added, "Some people in the community feel that they might lead to immune problems that could either exacerbate or lead to autism. All this is purely hypothetical, but this is an area where we need to continue to do more research."
To learn more about vaccines, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: Robert Schechter, M.D., medical officer, Immunization Branch, California Department of Public Health, Richmond; Andy Shih, Ph.D., vice president, scientific affairs, Autism Speaks, New York City; January 2008, Archives of General Psychiatry