Jan 30, 2008 -- Ethyl mercury from the vaccine preservative thimerosal leaves the blood 10 times faster than methyl mercury, on which current risk assessments are based.
Only trace levels of thimerosal can be found in U.S. vaccines, except in multi-dose vials of flu vaccine (single-dose flu vaccine has no thimerosal). But the inexpensive vaccines that allow poorer nations to afford to immunize their children still use thimerosal to prevent bacterial contamination.
Major studies of children who received thimerosal-preserved vaccines fail to find a link between these vaccines and health problems. But because thimerosal's active ingredient is a kind of mercury -- and because mercury can be highly toxic -- there is a belief among many parents of autistic children that thimerosal caused their children's disease.
Nearly everything known about the toxic effects of mercury is based on studies of a form of mercury called methyl mercury. That's the kind of mercury found in large ocean fish -- and the kind that causes developmental problems in children exposed to mercury through environmental disasters. But astonishingly little is known about the real risks of ethyl mercury itself.
A new study by University of Rochester researcher Michael E. Pichichero, MD, and colleagues now sheds some light on this mystery. Pichichero's team studied ethyl mercury levels in the blood, urine, and stools of Argentinean newborns and infants before and after they received multiple childhood immunizations with thimerosal-preserved vaccines.
"While our study is not a direct evaluation of neurological disorders and autism, it shows that mercury levels in infants' blood after vaccination with thimerosal-containing vaccines are 10 times lower -- and go away 10 times faster -- than if they'd received the same amount of methyl mercury," Pichichero tells WebMD.
One of the few researchers who studies the effects of ethyl mercury is Thomas Burbacher, PhD, professor of environmental and occupational health sciences and director of the infant primate research lab at the National Primate Research Center, University of Washington, Seattle. Burbacher's studies of ethyl mercury and thimerosal in primates are cited by both sides of the thimerosal debate.
Burbacher says that just because ethyl mercury is gone from an infant's blood soon after it receives a dose of thimerosal -- a half-life of just 3.7 days in the Pichichero study -- doesn't mean it's gone from the body.
"Just because it came out of the blood doesn't mean it is excreted from the body. It could have gone to the brain," Burbacher tells WebMD. "Although total mercury levels in the blood are lower following thimerosal exposure [than following methyl mercury exposure], mercury in the blood from thimerosal has an easier time getting to the brain than methyl mercury."
Pichichero agrees with Burbacher on this but he says the current findings are relevant to thimerosal risk.
"There is a direct relationship between the amount of mercury in the blood -- and how long it stays in the blood -- and the ability of mercury to get into the brain to produce developmental problems," he says. "We did not prove there was not deposition of mercury in other parts of the body, but we prove that the half-life of ethyl mercury from thimerosal is low, excretion is high, and the kidneys -- an organ very sensitive to the effects of mercury -- were not damaged."
Sallie Bernard is co-founder and executive director of SafeMinds, an organization that advocates "sensible action for ending mercury-induced neurological disorders."
Bernard says the Pichichero study is valuable because it makes it clear that the body handles ethyl mercury very differently from the way it handlesÂ methyl mercury.
"That is why we need real safety studies to look at the effects of ethyl mercury," Bernard tells WebMD. "We need not look at blood levels and excretion times, but at what goes on in the brain and what any ethyl mercury-induced changes in the brain are doing to children."
The Pichichero study appears in the February online edition of the journal Pediatrics.