A new study says that there is no link between mercury in vaccines and autism, writes Thomas Maugh
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 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 was metabolised by the body in much the same way as the methyl mercury found in fish and other sources. Current exposure guidelines are extrapolated from studies of methyl mercury.
But the first study of ethyl mercury in children shows that levels of mercury in the blood are only one-tenth as high as expected, and the toxic element is cleared from the blood very rapidly, according to a paper published yesterday 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, the lead author on the paper.
"Now it's obvious that ethyl mercury's short half-life prevents toxic buildup from occurring," he says. "It's just gone too fast."
The bottom line, says Dr William Schaffner of Vanderbilt University, who was not involved in the study, isthat "this is yet another study added to the increasing stack of studies that are reassuring about thimerosal's safety".
But Dr Isaac Pessah of the Mind Institute at the University of California, Davis, cautions that the researchers had studied only healthy children. They didn't address "the key issue of whether a subset of kids with metabolic disorders would handle it differently".
He also notes, as do the authors, that they were not able to examine the children's brains and other organs for mercury accumulation.
Still, the findings should be reassuring news for the parents of millions of infants who continue to receive vaccines containing thimerosal.
The preservative was eliminated from most childhood vaccines in the US in 1999, says Pichichero. But removing thimerosal from vaccines worldwide would raise prices enough to limit availability in the poorest countries.
Autism affects as many as one in every 167 children born in the US, and many parents continue to link the increase in cases to the widespread use of thimerosal in vaccines in the past. The new study was designed to address those concerns.
It confirms results previously obtained by 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 two-month-old infants and 72 six-month-olds at R Gutierrez Children's Hospital in Buenos Aires, Argentina, where thimerosal is still used in vaccines.
They found that blood levels of mercury 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 is the time it takes for half the mercury in blood to disappear. The half-life of methyl mercury, in contrast, is 44 days.
They also found that levels of mercury in the children's blood were about the same at birth, at two months and at six months.
"That's super-reassuring evidence that you don't accumulate mercury, you get rid of it," Schaffner says.
The researchers found no evidence of mercury in urine, indicating that the toxic metal was not coming into contact with the kidneys, which can be damaged by it. Most of the mercury, they found, was eliminated through faeces.
Dr Peter J Hotez of the Sabin Vaccine Institute at George Washington University, who was not involved in the study, characterises it as "beating a dead horse. I don't need this paper to tell me that there is no relationship between thimerosal and autism.
"On the other hand, it is useful to know that ethyl mercury does not have the same metabolism as methyl mercury," he says.
The study was funded by the National Institutes of Health. The researchers say they have in the past been paid for consulting with vaccine manufacturers.
- (Los Angeles Times/
Washington Post service)