Besides its proposed link to autism, the combination MMR vaccine contains thimerosal, a preservative that uses the toxic element mercury as an active ingredient.
Thimerosal is unrelated to the autism question.
But the FDA has said that in order to cut children's overall exposure to mercury, future lots of MMR vaccine will contain no thimerosal or only trace amounts. Dr. Karen Midthun, who directs FDA's office of vaccine research and review, told the committee that the agency lacks the authority to force a recall of vaccines unless it can verify that the product represents a clear and imminent threat to public safety. The FDA contends that the premarket testing required of all vaccines showed no increased risk of autism in children who got MMR shots. Midthun and others told the committee that the benefits of MMR and other childhood vaccines in preventing infectious diseases in children far outweighed the risk that the immunizations might be linked to autism. They said that pulling available MMR doses from the market would cause shortages in available vaccine and would also send unjustified panic throughout the public about the safety of immunizations.