WASHINGTON, Jan 30 (Reuters) - The mercury in a vaccine preservative is pumped out of a baby's body too quickly for it to do any damage, researchers reported on Wednesday in a study they say should further absolve shots of causing autism.

The study in the journal Pediatrics reinforces what many vaccine experts have said for years -- that the form of mercury found in the preservative is handled differently by the body than the kind found in pollution and contaminated fish.

"It takes a certain amount of time and a certain concentration ... to make it into the brain," Dr. Michael Pichichero of the University of Rochester in New York, who led the study, said in a telephone interview.

Thimerosal is widely used to preserve vaccines and is made from a kind of mercury called ethyl mercury. It allows vaccines to be distributed in multi-dose containers.

"The World Health Organization has continued to sanction the use of thimerosal in vaccines because it has proven to save many lives by preventing bacterial contamination," said Pichichero, who has served as a consultant to several vaccine manufacturers and the WHO.

Many studies have shown thimerosal is safe, but some vocal groups have continued to lobby against its use and it has been removed from all childhood vaccines in the United States except for some influenza vaccines.

Pichichero's team studied 216 infants from R. Gutierrez Children's Hospital in Buenos Aires, Argentina, where thimerosal is still routinely used in vaccines.

They were divided into three age groups to have their blood-mercury levels tested before and after they were immunized at their newborn, 2- or 6-month checkups.

Blood tests showed the half-life of ethyl mercury -- the time it takes for the body to dispose of half of it -- was 3.7 days. The more dangerous methyl mercury, known to damage nerves, has a half-life of 44 days.

GONE TOO FAST

"Until recently, that longer half-life was assumed to be the rule for both types of mercury," Pichichero said. "Now it's obvious that ethyl mercury's short half-life prevents toxic build-up from occurring. It's just gone too fast."

The blood tests also showed most of the infants had methyl mercury in their blood, at safe levels, before they were ever vaccinated, he said.

Autism is marked by a variety of difficulties in social interaction and behavior, from the awkwardness of Asperger syndrome to severely debilitating repetitive behaviors and an inability to speak.

The U.S. Centers for Disease Control and Prevention estimates that one in 150 children has autism or a related disorder in the United States.

Experts know genes play an important role. Because autism is often diagnosed in toddlers, who also have undergone a battery of childhood immunizations, some advocacy groups argue that the vaccines may have caused the condition.

The U.S. Institute of Medicine reported in 2004 that no link could be found between vaccines and autism and took the unusual step of urging researchers to look elsewhere for a cause.

The U.S. debate was renewed because ABC television is planning to debut a new legal drama this week with an episode in which a jury finds that a mercury-based preservative in a vaccine caused a child's autism.

The American Academy of Pediatrics asked ABC to cancel the show but the network has refused.

In a separate study on Wednesday, a team at the University of Washington's Autism Center reported that children with autism have normal-size heads at birth but develop accelerated head growth between six and nine months of age. (Editing by Will Dunham and John O'Callaghan)

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