|
medpundit |
||
 |
 |
|
Thursday, October 30, 2003Thimerosal has been used as a preservative in many vaccines since the 1930s. At concentrations found in vaccines, thimerosal meets the requirements for a preservative set forth by the U.S. Pharmacopeia — that is, it kills the specified challenge organisms and can prevent the growth of the challenge fungi. It contains the ethyl mercury radical (CH3CH2Hg+) attached to the sulfur group of thiosalicylate and is believed to behave toxicologically like other ethyl mercury compounds. Early toxicity studies found no adverse health effects; recently, however, Ball et al. reevaluated thimerosal by applying the revised EPA guideline for methyl mercury to ethyl mercury. They calculated that infants undergoing the usual U.S. program of vaccines from birth to six months of age would receive more than 0.1 µg of mercury per kilogram per day. Steps were rapidly taken to remove thimerosal from vaccines by switching to single-dose vials that did not require any preservative. This process is now virtually complete in the United States. The decision itself is remarkable, and the speed of execution even more so; however, the EPA guideline is based on epidemiologic data on prenatal exposure to methyl mercury rather than postnatal exposure to ethyl mercury. Ethyl mercury has some similarities to methyl mercury. They are closely related chemically, have a similar initial distribution in the body, and cause similar types of damage to the brain in toxic doses. They also have differences. Methyl mercury is more potent. Ethyl mercury is metabolized more rapidly to inorganic mercury; perhaps this is why ethyl mercury, unlike methyl mercury, causes kidney damage in humans. Of greater importance is the recent finding that the half-life of ethyl mercury in the body is much shorter . The half-life of methyl mercury in blood, which is assumed to indicate the total body burden, is usually assumed to be about 50 days. In contrast, in children receiving thimerosal in vaccines, the half-life of ethyl mercury in blood was 7 to 10 days, or 1/7 to 1/5 as long as that of methyl mercury. Therefore, in the two-month periods between vaccinations (at birth and at two, four, and six months), all of the mercury should have been excreted, so that there is no accumulation. Given the short half-life of ethyl mercury, any risks of its damaging either the brain or kidneys would seem remote. A World Health Organization advisory committee recently concluded that it is safe to continue using thimerosal in vaccines. This is especially important in developing countries, where the use of a preservative is essential in multidose vials. The known risk of infectious diseases far exceeds that of the hypothetical risk of thimerosal. Claims have been made that thimerosal in vaccines may be a cause of autism and related disorders, but studies testing that theory have yet to be performed. And the conclusion sums up the whole mercury brouhaha succinctly: All forms of mercury have adverse effects on health at high doses. However, the evidence that exposure to very low doses of mercury from fish consumption, the receipt of dental amalgams, or thimerosal in vaccines has adverse effects is open to wide interpretation. Moreover, attempts to reduce such exposure may pose greater health risks than those hypothesized to occur from mercury posted by Sydney on 10/30/2003 08:37:00 AM 0 comments 0 Comments: |
|
