MONDAY, Oct. 1, 2001 (HealthDayNews) -- A mercury preservative that until recently was commonly added to vaccines in this country does not appear to pose a risk of autism, hyperactivity and other neurological problems in children, according to a new report that looked at the possible risk.
The report, from the Institute of Medicine, which is an advisory panel to the government, failed to find a link between the additive thimerosal, found in vaccines and other drug products, and brain damage.
"I think [the report] should be reassuring that we could find no evidence of linking thimerosal to brain disorders," says Dr. Marie McCormick, a Harvard University public health researcher and chair of the panel. In addition, McCormick says, the evidence of even a potential connection is "quite indirect."
Still, the panel concluded that it was "biologically plausible" that cumulative exposure to thimerosal might make children vulnerable to mercury-related disorders. Existing population evidence "can't support or reject" a causal link between the chemical and harm to the brain, McCormick says.
So the report recommends that vaccine and other pharmaceutical makers that use thimerosal take steps to eliminate it from their products whenever possible. "The general sense is to reduce the exposure to mercury as much as possible," McCormick says. But if it's not possible, then a slight exposure to the chemical is "better than having bacterial contamination," she adds.
Thimerosal, which began to appear in multi-dose vaccine vials in the 1930s as a way to kill off potentially tainting germs, contains ethylmercury, a close but poorly understood cousin of the methylmercury produced by heavy industry and present in such foods as deep-water fish. Both forms of the element are toxic to brain cells in high doses. But it's not clear how much harm, if any, they cause in low amounts.
In 1999, the Public Health Service and two other medical groups called on vaccine makers to formulate a plan to remove thimerosal from their products, or at least reduce it. The FDA said at the time it would grant speedy review to companies that apply to sell mercury-free versions of their vaccines.
Since then, all of the childhood vaccines used in this country have become available in a thimerosal-free form. However, the preservative is still common in vaccines throughout the rest of the world, as well as in adult inoculations against influenza and diphtheria. The compound is also used in over-the-counter nose drops and other pharmaceutical products.
"It's far better to be vaccinated with a thimerosal-containing vaccine than to not be vaccinated," says McCormick.
Some evidence the panel reviewed showed that the battery of childhood vaccines might expose children to levels of mercury greater than those some government agencies deem safe. But Joshua Cohen, a researcher at Harvard's Center for Risk Analysis and another panelist, says that's neither true nor particularly concerning.
The alarming figures were based on moment-in-time looks at mercury levels, not the kind of prolonged exposure for which scientists have stronger data about harm from the toxin, he says.
"A more appropriate computation would suggest that the dose is, in fact, not vastly in excess of these federal" limits, if they eclipse them at all, Cohen says. What's more, he adds, "there isn't any direct evidence that injury occurs that those levels."
What To Do
To learn more about childhood immunization, check out the American Academy of Family Physicians, or the Centers for Disease Control and Prevention.
For more on the hazards of mercury and how to avoid the metal, try the Indiana Department of Environmental Management.
