Researchers from the state Department of Public Health found the autism rate in children rose continuously during the 12-year study period from 1995 to 2007. The preservative thimerosal hasn't been used in childhood vaccines since 2001, but is used in some flu shots.
Doctors say the latest study adds to existing evidence refuting a link between thimerosal exposure and autism risk and should reassure parents that the disorder is not caused by vaccinations. If there was a risk, they said, autism rates should have dropped between 2004 and 2007.
The findings show "no evidence of mercury poisoning in autism" since there was no decline in autism rates even after the elimination of thimerosal, said Dr. Eric Fombonne, an autism researcher at Montreal Children's Hospital who had no role in the research.
Some advocacy groups blame thimerosal for the impaired social interaction typical of autism. Nearly 5,000 claims alleging a vaccine-autism link have been lodged with the federal government, which is deciding whether victims should receive compensation from a government fund.
Dr. Daniel Geschwind, a neurologist at the David Geffen School of Medicine at the University of California, Los Angeles, said the focus now should be on exploring the causes of autism such as possible genetic links.
"Something else must be at play and we need to know what that is if we're really serious about preventing autism," said Geschwind, who had no connection with the study.
For their study, California public health officials calculated the autism rate by analyzing a database of state-funded centers that care for people with autism and other developmental disorders.
They found the prevalence of autism in children aged 3 to 12 increased throughout the study period. For example, 0.3 per 1,000 children born in 1993 had autism at age 3 compared with 1.3 per 1,000 children born in 2003. Similar trends were found in other age groups.
"These time trends are inconsistent with the hypothesis that thimerosal exposure is a primary cause of autism in California," the researchers wrote.
Results were published in January's issue of the journal Archives of General Psychiatry. The study did not explore why there was an increase in autism cases.
Federal statistics show about one in 150 children in the United States have autism, higher than previous estimates. Researchers say it's unclear if the increase is due to changes in how the disorder is classified or whether it's an actual spike.
Autism is characterized by impaired social interaction and communication skills. There is no cure, but early therapy can lessen the severity.