WASHINGTON (Reuters) -
The parents of two 10-year-old boys
who believe vaccines caused their sons to develop autism
brought their case to U.S. federal court on Monday, arguing a
mercury preservative in the shots caused a rare reaction.

Their case is the second of three being heard by a special
court trying to determine if autism might sometimes be caused
by vaccines. Although most medical experts say there is no
link, the court can rule there is a plausible association and
allow parents of children with autism to get federal
compensation from a special vaccine fund.

More than 5,300 cases have been filed by parents who
believe vaccines may have caused autism in their children and
are seeking payment under the National Vaccine Injury
Compensation Program, a no-fault system that pays out for
vaccine injuries.

Under the program, someone injured by a vaccine does not
have to prove the vaccine actually caused his or her injuries.

All that is necessary is to prove that vaccines sometimes
cause that particular condition or injury. The no-fault payout
system is meant to protect vaccine makers from costly lawsuits
that drove many out of the vaccine-making business.

Payouts in such cases sometimes top $1 million. The
compensation program is funded by a 75-cent tax on
vaccinations.

The court is hearing three different theories on how
vaccines might cause autism. One is that a combination vaccine
for measles, mumps and rubella, or MMR, plus a
mercury-containing preservative called thimerosal, can cause
autism. The court heard those arguments last year and has not
ruled.

On Monday, the court began hearing arguments that
thimerosal in various vaccines might have caused autism in
William Mead and Jordan King, both 10 and both from Portland,
Oregon.

INTERACTION WITH GENES

“What we will conclude … is that thimerosal-containing
vaccines belong on the list of environmental factors … when
one is evaluating what might have caused autism in a child when
all of the other theories have been ruled out,” attorney Tom
Powers told the court in opening arguments.

He said the boys had conditions that made them especially
vulnerable to the mercury in thimerosal.

“The evidence is indirect and it is circumstantial but it
is supportive of the general theory of causation,” he said.

No one knows what causes autism, which can severely disable
a child with symptoms ranging from severe social avoidance to
repetitive behaviors and sometimes profound mental retardation.

The U.S. Centers for Disease Control and Prevention
estimates that about one in every 150 children has autism or a
related disorder such as Asperger's syndrome.

Doctors agree there is a genetic link, and probably that
something in the environment, possibly even conditions in the
womb, can cause the brain effects that lead to symptoms.

While many studies have shown the thimerosal in vaccines
has not caused autism, a vocal group argues the government and
other experts are ignoring or covering up the evidence.
Thimerosal has now been removed from most childhood vaccines.

“I think we will be able to convince you that the epidemic
is real, that the increase is real,” Mike Williams, another
attorney for the boys, told the three “special masters” hearing
the cases.

“The debate is over. There is no controversy,” government
attorney Lynn Ricciardella retorted in her opening arguments.

“The credible scientific community has already spoken on
this issue and has rejected it.”

Some autism activists have seized on the case of Hannah
Poling, a girl from Georgia who won a case claiming a vaccine
caused autism-like complications from a rare disorder.

The activists say it proves the federal courts gave in on
the argument, but the government says Poling's case, which was

removed from the special process and heard separately, was
an exception and cannot be used as a precedent.

(Editing by Will Dunham and Cynthia Osterman)