In a big blow to parents who believe vaccines caused their children's autism, a special court in the U.S. ruled Thursday that the shots are not to blame.

The judges in the cases said the evidence was overwhelmingly contrary to the parents' claims — and backed years of science that found no risk.

"It was abundantly clear that petitioners' theories of causation were speculative and unpersuasive," the court concluded in one of a trio of cases ruled on Thursday.

The ruling was anxiously awaited by health authorities and families who began presenting evidence in June 2007. More than 5,500 claims have been filed by families seeking compensation through the government's Vaccine Injury Compensation Program.

The claims are reviewed by special masters serving on the U.S. Court of Claims.

"Hopefully, the determination by the special masters will help reassure parents that vaccines do not cause autism," the Department of Health and Human Services said in a statement.

An attorney for the families did not respond immediately to a request for comment. But the head of one consumer group that questions vaccine safety, the National Vaccine Information Center, said the court's ruling will do little to change the minds of most parents who suspect a link between vaccines and autism.

She said more studies are needed.

"I think it is a mistake to conclude that, because these few test cases were denied compensation, it's been decided vaccines don't play any role in regressive autism," said Barbara Loe Fisher, the centre's president.

To win, the families' attorneys had to show that it was more likely than not that the autism symptoms in the children were directly related to a combination of the measles-mumps-rubella shots and other shots that at the time carried a mercury-containing preservative called thimerosal.

But the court concluded that "the weight of scientific research and authority" was "simply more persuasive on nearly every point in contention."

"It's a great day for science, it's a great day for America's children when the court rules in favour of science," said Dr. Paul Offit of the Children's Hospital of Philadelphia.

The court still has to rule on separate claims from other families who contend that, rather than a specific vaccine combination, the lone culprit could be thimserosal, a preservative that is no longer in most routine children's vaccines.

But in Thursday's rulings, the court may have sent a signal on those cases, too: "The petitioners have failed to demonstrate that thimerosal-containing vaccines can contribute to causing immune dysfunction," a judge wrote about one theory that the families proposed to explain how autism might be linked.