On Wednesday, Robert F. Kennedy Jr., along with actor Robert de Niro, staged a press conference to announce a $100,000 prize to anyone who could prove vaccines are safe. The prize is sponsored by Kennedy's organization, the World Mercury Project, an anti-vaccine group "determined to create a world free of the devastating effects of mercury."

According to the organization's website, the WMP "will pay $100,000 to the first journalist, or other individual, who can find a peer-reviewed scientific study" proving that vaccines are safe. Specifically, they are concerned with thimerosal, a compound used in some vaccines as a preservative. Of course, the CDC has links to a dozen studies specifically focusing on the effects of thimerosal in children, which Kennedy's group has already dismissed on spurious grounds. Still, it can't hurt to go through the evidence.

Thimerosal is a mercury-based compound used mainly as a preservative in the flu vaccine, although it used to be found in many others. Concerns about the safely of thimerosal arose in the 1990s, prompting the compound to be removed from most vaccines. Since then, numerous studies have failed to find any evidence that thimerosal levels in vaccines are harmful in any way.

It's true that at high concentrations, thimerosal is toxic. Just like any other mercury compound, it can cause a number of neurological conditions and birth defects. However, a typical vaccine contains between 25 and 50 micrograms of thimerosal, and a report by the FDA found that harmful effects only occurred at levels 100 times that.

More importantly, thimerosal doesn't stay in the body long enough to accumulate. Unlike other kinds of mercury, thimerosal is easily metabolized by your body and passes safely in about a week. In the time it would take to inject yourself with enough vaccines to reach a harmful dose of thimerosal, most of it would already have been excreted.

There have been dozens of studies that fail to show a link between thimerosal and autism or any other condition. Even the WMP can only point to a handful of weak correlations as evidence of thimerosal's supposed danger. These correlations are more likely to be statistical flukes than proof that thimerosal causes anything.

But if the WMP's challenge is to find a single peer-reviewed study showing that thimerosal is safe at vaccine levels, here's an FDA study from almost 20 years ago that "revealed no evidence of harm caused by doses of thimerosal in vaccines." For extra credit, you could look at the fact that thimerosal was eliminated from childhood vaccines more than 15 years ago, yet the autism rate in the U.S. continues to rise. No matter how you look at the data, the conclusion is the same: thimerosal is safe.

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