WASHINGTON (AP) - Parents claiming that childhood vaccines cause autism should not be rewarded by the courts when the scientific community has already rejected any link, government lawyers argued Monday on the first day of a hearing in federal court.

Overall, nearly 4,900 families have filed claims with the U.S. Court of Claims alleging that vaccines caused autism and other neurological problems in their children. Lawyers for the families are presenting three different theories of how vaccines caused autism. The theory at issue Monday was whether vaccines containing the preservative thimerosal caused autism.

Lynn Ricciardella, a Justice Department lawyer, said that theory has not moved beyond the realm of speculation. She said that the Institute of Medicine and the Centers for Disease Control and Prevention have rejected any link between thimerosal and autism.

"There is no scientific debate," Ricciardella said. "The debate is over."

Autism is a developmental disability that typically appears during the first three years of life and affects a person's ability to communicate and interact with others. Medical experts don't have a comprehensive understanding of what causes autism, but they do know there is a strong hereditary component.

Thimerosal has been removed in recent years from standard childhood vaccines, except flu vaccines that are not packaged in single doses. The CDC says single-dose flu shots currently are available only in limited quantities.

Under a two-decades-old program, individuals claiming injury from a vaccine must file a petition for "no-fault" compensation with the U.S. Court of Federal Claims. The secretary of Health and Human Services replaces the vaccine manufacturer or vaccine administrator to defend the claim.

Two 10-year-old boys from Portland, Ore., will serve as test cases to determine whether thousands of families can be compensated. Attorneys for the boys will try to show they were happy, healthy and developing normally - but, after being exposed to vaccines with thimerosal, they began to regress.

To win, the attorneys for the two boys, William Mead and Jordan King, will have to show that it's more likely than not that the vaccine actually caused the injury, which they described as regressive autism.

Tom Powers, one of the boys' attorneys, acknowledged that the evidence showing thimerosal led to regressive autism was indirect and circumstantial. Still, it's clear in the case of the two boys that they did not show any symptoms of autism until after they had received all their immunizations.

"Each of them had developed normally and typically well after their first year in life," Powers said.

The attorneys for the two boys said that a study in monkeys showed that mercury could ignite "neuroinflammation" in the brain, and such inflammation is the hallmark of somebody with autism. They also noted that previous studies of thimerosal were focused on autism, rather than on a more rare, specific form of the disorder that they described as regressive autism.

The first witness for the families, Sander Greenland, a professor at the UCLA School of Public Health, said published studies he reviewed failed to separate regressive autism from other types of autism when looking at thimerosal, thus they allow for a substantial association of the vaccines with clearly regressive autism.

Under the vaccine compensation program, officials titled special masters serve as the trial judges. The hearing that began Monday involved three special masters who will hear the evidence and determine whether thimerosal belongs on the list of causes for regressive autism. The rulings are appealable to the Court of Federal Claims.

If the families are successful, they could be entitled to damages that cover lost income after one turns 18 and up to $250,000 for pain and suffering.

Many members of the medical community are skeptical of the families' claims. They worry that the claims about the dangers of vaccines could cause some people to forgo vaccines that prevent illness.

Ricciardella argued that a marketing consultant fanned publicity about the supposed link between thimerosal and autism in a journal called Medical Hypothesis. She described the journal as willing to publish radical ideas, so long as they are coherent. She also said the authors pay to have the article published.

But Powers said those questioning conventional wisdom in the case cannot be easily dismissed.

"These are doctors who are willing to challenge the establishment on behalf of their patients," Powers said.

The court Web site says more than 12,500 claims have been filed since creation of the program in 1987, including more than 5,300 autism cases, and more than $1.7 billion has been paid in claims. It says there is now more than $2.7 billion in a trust fund supported by an excise tax on each dose of vaccine covered by the program.

On the Net:

Background on thimerosal trial: http://www.uscfc.uscourts.gov/node/4428