* Ruling is the fourth against autism claims

* Court says mercury preservative did not cause autism

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.

The Meads had filed a civil lawsuit in Oregon state court against a number of pharmaceutical companies alleging that the thimerosal additive in many pediatric vaccines significantly contributed to the development of William’s autism, Hastings wrote.

While the state court determined the autism was vaccine-related, Hastings said overwhelming medical evidence showed otherwise. The theory presented by the Meads and experts who testified on their behalf “was biologically implausible and scientifically unsupported”, Hasting wrote.

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.

In a separate matter, the U.S. Supreme Court said earlier this month it would decide whether a federal law protects vaccine manufacturers from lawsuits in state court seeking damages for alleged design defects.

The high court agreed to hear a Pennsylvania case involving a lawsuit by the parents of a child who suffered seizures after her third dose of a diphtheria-tetanus-pertussis vaccine. They sued the vaccine manufacturer, Wyeth, which Pfizer Inc PFE.N purchased last year.