Thimerosal and vaccines--a cautionary tale.

n engl j med 357;13 www.nejm.org september 27, 2007 1278 I 1997, Frank Pallone, a U.S. congressman from New Jersey, attached a simple, 133-word amendment to a Food and Drug Administration (FDA) reauthorization bill. This amendment gave the FDA 2 years to “compile a list of drugs and foods that contain intentionally introduced mercury compounds and [to] provide a quantitative and qualitative analysis of the mercury compounds in the list.”1 The bill — the FDA Modernization Act of 1997 — was signed into law on November 21, 1997. Neither the press nor the public took notice. Eighteen months later, in May 1999, the FDA found that by 6 months of age, infants could receive as much as 75 μg of mercury from three doses of the diphtheria–tetanus–pertussis vaccine, 75 μg from three doses of the Haemophilus inf luenzae type b vaccine, and 37.5 μg from three doses of the hepatitis B vaccine — a total of 187.5 μg of mercury. The use of mercury in vaccines wasn’t new; thimerosal, an ethylmercury-containing preservative, had been used to prevent bacterial contamination since the 1930s. To determine whether the amount of mercury in vaccines was safe, FDA scientists examined safety guidelines from three sources: their own agency, the Environmental Protection Agency, and the Agency for Toxic Substances and Disease Registry. They found safety guidelines for methylmercury (environmental mercury), but not for ethylmercury (thimerosal). Although these two molecules differ by only one carbon atom, the difference isn’t trivial. Ethylmercury is excreted from the body much more quickly than methylmercury and is therefore much