New studies in infants show that the mercury used as a preservative in vaccines is cleared from the body at least 10 times faster than researchers had previously believed, a finding that casts further doubt on the theory that the preservative causes autism.
Researchers had believed that the ethyl mercury in the preservative thimerosal is metabolized in much the same way as the methyl mercury found in fish and other sources.
But the first study of ethyl mercury in children shows that levels of mercury in the blood are only a tenth as high as expected, and the toxic element is cleared out rapidly, according to a paper to be published Monday in the journal Pediatrics.
There is a "clear relationship" between the amount of mercury that must be in the blood, the length of time it must remain there, and the likelihood of it accumulating in the brain to cause damage, said Dr. Michael E. Pichichero of the University of Rochester in New York, the paper's lead author. "Now it's obvious that ethyl mercury's short half-life prevents toxic buildup from occurring. It's just gone too fast."
The bottom line, said Dr. William Schaffner of Vanderbilt University, who was not involved in the study, is that "this is yet another study added to the increasing stack of studies that are reassuring about thimerosal's safety."
But Isaac Pessah of the UC Davis MIND Institute pointed out that the researchers had only studied healthy children. They didn't address "the key issue of whether a subset of kids with metabolic disorders would handle it differently."
Like the authors, he also noted that they couldn't examine the brain and other organs for mercury accumulation.
Still, the findings should reassure parents of millions of infants around the world who receive vaccines with thimerosal even though it was eliminated from most childhood vaccines in the U.S. in 1999, Pichichero said. Removing thimerosal would raise prices and limit availability in poor countries.
Autism strikes as many as one in 167 children born in the U.S. Many parents link the increase in cases to past use of thimerosal in vaccines. The new study was designed to address those concerns.
It confirms previous findings of Pichichero and his colleagues in studies in rhesus monkeys and in a much smaller group of infants.
In the latest study, they examined 72 newborns, 72 2-month-old infants and 72 6-month-olds at R. Gutierrez Children's Hospital in Buenos Aires, where thimerosal is still used in vaccinations.
They found that blood mercury levels spiked shortly after vaccination -- although they remained much lower than levels of methyl mercury observed in other studies -- then dropped, with a half-life of 3.7 days. The half-life of methyl mercury, in contrast, is 44 days.
They also found that levels of mercury in the blood were about the same at birth, at 2 months and at 6 months.
"That's super-reassuring evidence that you don't accumulate mercury, you get rid of it," Schaffner said.
The researchers found no evidence of mercury in urine, indicating that the toxic metal was not coming into contact with the kidneys. Most of the mercury, they found, was eliminated through the feces.
Dr. Peter Hotez of the Sabin Vaccine Institute, who was not involved in the study, characterized it as "beating a dead horse."
"On the other hand, it is useful to know that ethyl mercury does not have the same metabolism as methyl mercury," he said.
The study was funded by the National Institutes of Health. The researchers said they had in the past been paid for consulting with vaccine manufacturers.