After assuring parents that additive in vaccines don't cause brain damage, scientists have found what they believe could be a "smoking gun" linking these additives to autism and attention-deficit hyperactivity disorder in children.

In a study that was rushed to print online today, two months ahead of its scheduled publication in the journal Molecular Psychiatry, U.S. researchers have discovered an apparent link between thimerosal, a controversial mercury-based preservative once commonly used in childhood vaccines, to an increased risk of neurological disorders such as autism and ADHD.

While most vaccines distributed in Canada have been thimerosal-free since the early 1960's, the preservative was used in the annual flu shot that doctors recommended this year for even healthy children.

In tests on human brain cells, researchers found two natural chemicals; one compound that simulates cell growth and also dopamine, which transmits nerve signals, both are key to a process in the brain called methylation. Methylation helps DNA work properly and is crucial to the normal development of the brain.

The team found thimerosal, ethanol and the metals lead and mercury all interfere with methylation. What's more, thimerosal not only did so in amounts typically found after a child is vaccinated, but even at doses 100 times lower than a child could ever receive after a single shot with a thimerosal containing vaccine.

"It was by far the most potent," says investigator Dr. Richard Deth, a professor of pharmacology at Northeastern University in Boston.

He said the study, which also involved researchers from Johns Hopkins University, the University of Nebraska and the Tufts University in Boston, could account for the rising rates of autism since the early 1980's, when more thimerosal-containing shots were added to a child's vaccine schedule.

A recent review of vaccine-related "adverse events" in the U.S. found a "significant correlation" between shots containing thimerosal and autism, the researchers report.

Thimerosal had been used to prevent the growth of bacteria or fungi in the multi-dose units of vaccines for diseases such as hepatitis and diphtheria, pertussis (whooping cough) and tetanus, or DPT.

But one of Canada's leading experts in vaccination says large studies have repeatedly failed to find any association between brain damage and vaccines that do, or don't, contain thimerosal.

"What (the researchers) are doing in the test tube may or may not have any relationship to what happens in the body," added Dr. Ronald Gold, professor emeritus of pediatrics at that University of Toronto and author of Your Child's Best Shot: A Parents Guide to Vaccination. He said that there's no evidence that the low doses of thimerosal researchers tested would even cross a child's blood-brain barrier.

But Deth thinks there may be a link, and he believes thimerosal may play a role for the one out of 200 children who will experience some kind of developmental disorder.

Before the early 1990's most causes of autism were believed to have a strong genetic component, and symptoms surfaced soon after the child was born.