CHICAGO, Jan 7 (Reuters) - A new study provides more proof that childhood vaccines with mercury as a preservative -- no longer on the market -- did not cause autism, researchers reported on Monday.

The findings came from a look at children diagnosed with autism in California from 1995 to 2007. It found that the number of autism cases continued to rise through that period even though the preservative thimerosal -- nearly half of which is made of ethylmercury -- was removed from most vaccines in 2001.

The data "do not show any recent decrease in autism in California despite the exclusion of more than trace levels of thimerosal from nearly all childhood vaccines (and) do not support the hypothesis that that exposure (to it) during childhood is a primary cause of autism," the study concluded.

Some earlier studies had linked mercury to autism, theorizing that as more and more children were being vaccinated against more health threats, it could explain increases in autism.

But a 2004 report from the U.S. Institute of Medicine said a review of existing studies did not appear to back the mercury-autism theory.

What causes autism remains a mystery. Some experts have said the increased number of cases is due at least in part to more awareness, an expanded definition, education and other factors.

People with autism spectrum disorders suffer in varying degrees from limited social interactions, lack of verbal and nonverbal communication and other abilities. As many as 1.5 million people in the United States have some form of autism, which is generally diagnosed beyond the age of 2, after most vaccinations have occurred.

"Although our analysis ... shows an increase in autism in California despite the removal of thimerosal from most vaccines, we support the continued quest for the timely discovery of modifiable risk factors for autism and related conditions," said the report from Dr. Robert Schechter and Judith Grether of the California Department of Public Health.

The report was published in the Archives of General Psychiatry.

In an editorial in the same publication commenting on the findings, Dr. Eric Fombonne of Montreal Children's Hospital, said that despite this study and earlier ones, beliefs persist among the general population that autism is related to mercury-preserved vaccines or to the triple measles-mumps-rubella vaccine specifically.

But, he said, parents of autistic children "should be reassured that autism in their child did not occur through immunizations."

"Their autistic children, and their siblings, should be normally vaccinated, and as there is no evidence of mercury poisoning in autism, they should avoid ineffective and dangerous 'treatments' such as chelation therapy ..." he added. (Reporting by Michael Conlon; Editing by Doina Chiacu)

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