Infants expel the mercury contained in a common vaccine preservative too quickly for toxicity to build up, US researchers said, saying their findings should allay fears that child vaccines can cause autism.
A study conducted at the University of Rochester in New York in the medical journal Pediatrics showed that infants "expel thimerosal mercury much faster than originally thought, thereby leaving little chance for a progressive building up of the toxic metal."
"This debunks the great myth, believed by both parents and some pediatricians, that the gauntlet of thimerosal-containing shots many infants received in the 1990s, when the average number of vaccines kids received increased sharply, had put them at risk for developmental disorders," including autism, a statement issued by the university said.
About 5 000 US families last year brought a case before a special US court, which is still ongoing, seeking to prove that vaccines administered to children are linked to a rise in the incidence of autism, which now affects as many as one child in every 150 in the United States.
But the California Department of Health last month reported that autism rates continue to mushroom in the United States, even though thimerosal was removed from most child vaccines in 2001 at the urging of health officials and the American Academy of Pediatrics (AAP), concerned over its mercury content.
The fears surrounding thimerosal arose in the late 1990s when it was found that children who received "a complete series of vaccines that contained thimerosal potentially received up to 187,5 grams of ethyl mercury during the first six months of life," the Rochester study said.
That vastly exceeded "the US Environmental Protection Agency's recommended safe intake level, estimated in 1997 to be no more than 0,1 grams of mercury per kilogram of body weight per day," it said.
But the Environmental Protection Agency guidelines were based on oral methyl mercury, the type associated with eating fish, not the intramuscular ethyl mercury found in thimerosal.
"Scientists are learning that the two mercury species actually behave quite differently... the body rids the kind found in thimerosal more that 10 times faster than it removes the kind one might encounter in a Friday night fish fry," the Rochester researchers said in a statement.
For the Rochester study, the blood mercury levels of 216 infants at a hospital in Argentina were tested before and after shots were administered at either their newborn, two- or six-month checkup.
Thimerosal is still widely used in childhood vaccines outside the United States.
The study found the half-life of ethyl mercury in the blood - or the time it takes for the body to dispose of half the mercury, then another half, and so on until it is gone - to be 3,7 days.
"That's a far cry from the blood half-life of methyl mercury, which is 44 days," the researchers said.
"Until recently, the longer half-life was assumed to be the rule for both types of mercury. Now it's obvious that ethyl mercury's short half-life prevents toxic build-up from occurring. It's just gone too fast," said the lead author of the study, Dr Michael Pichichero.
The AAP has called on US television network ABC to cancel the opening episode of a show called Eli Stone, which features a lawyer arguing in court that a vaccine caused a child's autism.
"A television show that perpetuates the myth that vaccines cause autism is the height of reckless irresponsibility on the part of ABC and its parent company, The Walt Disney Company," AAP president Renee Jenkins said in a statement.
"If parents watch this programme and choose to deny their children immunisations, ABC will share in the responsibility for the suffering and deaths that occur as a result," she said.
The television network had not responded to the demand on the eve of the airing, AAP spokesperson Debbie Linchesky said.