The finding adds to a growing body of evidence against the " thimerosal hypothesis." The hypothesis holds that vaccines using thimerosal as a preservative -- given to children or their mothers -- causes autism.
As a precautionary measure, by March 2001 thimerosal was removed (except for trace amounts) from all recommended infant vaccines. The only childhood vaccines that still contain more than trace amounts of thimerosal are multiple-dose vials of some flu vaccines.
An early look at data from California suggested that removal of thimerosal might have led to a decline in autism. But now an updated look at the data by the California Department of Public Health shows that there was no such decline.
"We are reassured that we found no link between routine childhood vaccination and increases in childhood autism in the data," California DPH Medical Officer Robert Schechter, MD, tells WebMD. "But the finding that there are increasing numbers of kids who need services is not reassuring. We support efforts to find preventable causes of autism."
The data are based on children referred to the California Department of Developmental Services System, a network of regional centers that serve people disabled by autism, mental retardation, and other developmental disabilities.
The most recent data come from March 2007, reflecting autism in children as young as age 3. Data on children younger than 3 are not routinely entered into the system. One 2001 look at the data by proponents of the thimerosal hypothesis suggested that there was a decline in autism beginning in 1994. A year earlier, a thimerosal-free Hib vaccine reduced the amount of thimerosal to which children were exposed.
But Schechter says there was no such decline.
"Not enough time had passed for autism in the younger children in that time period to be fully recognized," he says. "We saw with additional time, rather than going up and going down, autism rates continued to go up."
Autism Rise: Is It Something in the Environment?
It may still be too soon to draw conclusions from the California data, says Sally Bernard, co-founder and executive director of SafeMinds, an organization that advocates "sensible action for ending mercury-induced neurological disorders."
"Their conclusions rely on the younger age group of 3- to 5-year-olds," Bernard tells WebMD. "So if you are looking at trends, it is premature to make such conclusions."
Bernard is far less concerned about what the data do not show than about what they do show: autism rates are still going up.
"This tells us that because autism is steadily increasing, there is an environmental source," she says. "You cannot use this data to rule out thimerosal as a factor. What you can say is there may be -- and probably are -- other environmental triggers that play a role. It is probable that exposure to these other factors may be increasing."
If thimerosal does play a role in autism, it isn't a big one, says Gary W. Goldstein, MD, president and CEO of Baltimore's Kennedy Krieger Institute.
"The thimerosal is pretty much gone now, and autism rates are still going up," Goldstein tells WebMD. "It just is not the case that thimerosal in vaccines is the primary cause for the increase in autism. Could there be 5% of children more susceptible to autism and the thimerosal just tipped them over the edge? Yes, but this study did not address that question."
The Schechter study appears in the January issue of Archives of General Psychiatry.