* Ruling is the fourth against autism claims

* Court says mercury preservative did not cause autism (Adds reaction)

WASHINGTON, March 12 (Reuters) - Vaccines that contain a mercury-based preservative called thimerosal cannot cause autism on their own, a special U.S. court ruled on Friday, dealing one more blow to parents seeking to blame vaccines for their children’s illness.

The special U.S. Court of Federal Claims ruled that vaccines could not have caused the autism of an Oregon boy, William Mead, ending his family’s quest for reimbursement.

“The Meads believe that thimerosal-containing vaccines caused William’s regressive autism. As explained below, the undersigned finds that the Meads have not presented a scientifically sound theory,” Special Master George Hastings, a former tax claims expert at the Department of Justice, wrote in his ruling.

In February 2009, the court ruled against three families who claimed vaccines caused their children’s autism, saying they had been “misled by physicians who are guilty, in my view, of gross medical misjudgment”.

The families sought payment under the National Vaccine Injury Compensation Program, a no-fault system that has a $2.5 billion fund built up from a 75-cent-per-dose tax on vaccines.

Instead of judges, three “special masters” heard the three test cases representing thousands of other petitioners.

They asked whether a combination vaccine for measles, mumps and rubella, or MMR, plus a mercury-containing preservative called thimerosal, caused the children’s symptoms.

MYSTERIOUS CONDITION

More than 5,300 cases were filed by parents who believed vaccines may have caused autism in their children. The no-fault payout system is meant to protect vaccine makers from costly lawsuits that drove many out of the vaccine-making business.

Autism is a mysterious condition that affects as many as one in 110 U.S. children. The so-called spectrum ranges from mild Asperger’s Syndrome to severe mental retardation and social disability, and there is no cure or good treatment.

The U.S. Institute of Medicine has reported several times that no link can be found between vaccines and autism.

Supporters of the scientific community welcomed the ruling.

“It’s time to move forward and look for the real causes of autism,” said Alison Singer, president of the Autism Science Foundation. “There is not a bottomless pit of money with which to fund autism science. We have to use our scarce resources wisely.”

But advocates for the idea that vaccines are dangerous said they would not give up. “We hope that Congress will intervene in what is clearly a miscarriage of justice to vaccine-injured children,” said Jim Moody of the Coalition for Vaccine Safety.

Autism Speaks, another advocacy group, said it would also not completely abandon the theory that vaccines might cause autism.

The organization said it would invest “in research to determine whether subsets of individuals might be at increased risk for developing autism symptoms following vaccination.”

But the group also said it was clear that if such a link did exist, it would be rare.

“While we have great empathy for all parents of children with autism, it is important to keep in mind that, given the present state of the science, the proven benefits of vaccinating a child to protect them against serious diseases far outweigh the hypothesized risk that vaccinations might cause autism,” Autism Speaks said in a statement.