After hearing the warnings about the mercury we ingest when eating fish and how toxic heavy metals are, it might seem like a bad idea to use mercury in vaccines that are injected into the human body.
But scientists say that the mercury used with some vaccines -- known as ethyl mercury or thimerosal -- is much different than the methyl mercury that comes from fish.
Methyl mercury can accumulate in body tissues. At high levels it has been shown to cause such mercury poisoning symptoms as tremors, muscle weakness, memory loss and hypersensitivity to light.
But researchers have found no evidence that ethyl mercury in vaccines causes such problems, and they have found no link between thimerosal and autism, despite persistent arguments by some parents of autistic children that there is a connection.
"This myth was originally created by a couple of mothers of autistic children who read about methyl mercury poisoning and thought it sounded like autism," said Dr. Martin Myers, director of the National Network for Immunization Information and a pediatrics professor at the University of Texas at Galveston.
Thimerosal had been used since the 1930s as a preservative in vaccines and other biological preparations, to keep multi-dose vials from being contaminated by repeated insertions of needles. Doctors believed it was highly beneficial, Myers said, "because there had been some terrible earlier tragedies of children getting vaccines that were contaminated and then dying."
Despite the lack of evidence that thimerosal was causing harm, manufacturers agreed to remove it as a preservative from almost all childhood vaccines starting in 1999.
By January 2002, Myers said, the last vials containing thimerosal preservative expired. The only common immunizations that have it are Fluzone and Fluvirin, which also come in versions without thimerosal.
Manufacturers got around the problem of removing thimerosal either by using alternative preservatives or by creating single-dose vaccines that didn't need a preservative.
In a couple of cases, thimerosal is used by manufacturers to prevent contamination during preparation, but it is then removed before the final product is packaged.
The Food and Drug Administration lists those vaccines, including one version of diphtheria-pertussis-tetanus vaccine, as having a trace of thimerosal, because the extraction process can't remove 100 percent of it.
Dr. Neal Halsey, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health in Baltimore, was one of the people who pushed for an investigation of thimerosal in vaccines in the late 1990s.
"We weren't concerned about autism," he said. "... There was a concern, though, that if by chance children got multiple thimerosal-containing vaccines, they might have enough mercury exposure that it might exceed the EPA guidelines."
The research showed thimerosal was excreted from children's bodies much more quickly than methyl mercury.
Historically, there is only one case where ethyl mercury has been removed from the American market because of real risks, Myers added. That involved the two antiseptics commonly used on baby-boomer children -- Mercurochrome and Merthiolate.
In a few cases, Myers said, families put so much mercury compound on wounds that they created mild cases of poisoning. The products were pulled from the domestic market several years ago.
Halsey said it is understandable for some parents to associate vaccines with autism if their children's symptoms began around the time they received their immunizations.
He faults organizations like Autism Speaks on that issue, because "they're not explaining that there will be coincidental timing of people recognizing the early signs of autism at the same time vaccines are given, so it's an impossible task to try to keep doing research that addresses concerns that parents raise about bad things that are temporally associated with autism."
(Contact Mark Roth at mroth(at)post-gazette.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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