CHICAGO (AP) _ Autism rates in Denmark do not appear to be linked to thimerosal, a mercury-containing preservative once added to some childhood vaccines, according to an analysis of three decades of data.
An apparent increase in autism rates in Denmark began shortly before the discontinuation of thimerosal-containing vaccines there in 1992 but continued for several years thereafter, the study found.
``Thimerosal has been eliminated from childhood vaccines in most industrialized countries,'' said lead author Dr. Kreesten Meldgaard Madsen. ``If indeed thimerosal was an important cause of autism, (autism rates) should soon begin to decline in these countries.''
``We did not see this decline,'' said Madsen, whose study was published Tuesday in the September issue of Pediatrics.
Though the amount of mercury in vaccines was small, vaccine makers in the United States began phasing out thimerosal a few years ago as a precaution recommended by public health officials.
Mercury can cause neurological damage in high doses.
Many parents of autistic children think increases in the number of recommended childhood vaccines are to blame for the apparent autism surge, though many scientists think that's just a coincidence.
The Institute of Medicine reviewed the issue and in 2001 said a potential link between thimerosal and neurodevelopmental disorders was unproven but medically plausible. The institute, a private advisory group to the U.S. government, recommended additional research.
The Danish researchers examined data on 956 children diagnosed with autism from 1971 to 2000. They said the autism incidence rate climbed steadily from less than one child per 10,000 in 1990 to nearly 5 per 10,000 in 1999, seven years after thimerosal was removed from vaccines in Denmark.
Dr. Robert Byrd of the University of California, Davis, who has studied a surge in autism cases in California, said the Danish study won't settle the question.
Its flaws include using only data on hospitalized autistic children until 1995 but adding outpatients after that, which muddles whether there were any changes in autism rates, Davis said.
If thimerosal is really a culprit, ``it needs to be addressed,'' Byrd said. ``If it's not real, it still needs to be addressed so attention focused on vaccines can be directed toward'' potential causes.
Autism is a developmental disorder featuring a spectrum of symptoms including impairments in communication and social interaction and repetitive behaviors such as finger tapping or head banging.
Recent data suggest it might affect at least 40 per 10,000 U.S. children, 10 times higher than estimates a decade ago.
Many scientists think that reflects better recognition of the disorder, though some believe it represents a true increase, possibly due to environmental toxins or other factors. Most think genetics and environmental factors play a role, but the exact cause is unknown.
Mark Blaxill, a director of the Safe Minds activist group, said it's not surprising that Pediatrics would publish a pro-vaccine study since the journal's core readers _ pediatricians _ administer vaccines.
Safe Minds was founded by parents of autistic children seeking to raise awareness about the risks of mercury, and many of its members believe thimerosal is to blame.