A special vaccine court ruled on Thursday that vaccines are not to blame for children's neurological disorder.
The judges determined that there was overwhelming evidence against the parents' claims, saying that years of science had found no risk.
"It was abundantly clear that petitioners' theories of causation were speculative and unpersuasive," the court concluded.
The ruling was a blow to over 5,000 families that have filed claims for compensation through the governments Vaccine Injury Compensation Program.
The court said that "the weight of scientific research and authority" was "simply more persuasive on nearly every point in contention."
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