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.

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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.

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