MONDAY, Dec. 17 (HealthDay News) -- A United Nations proposal to ban the vaccine preservative thimerosal -- which contains a form of mercury -- should not go through, says a leading group of U.S. pediatricians.

The American Academy of Pediatrics (AAP) joins the World Health Organization (WHO) in urging the U.N. to drop the proposal from an international treaty seeking to cut down on mercury exposures in a variety of ways. Both the WHO and AAP say a thimerosal ban could keep children in poor nations from getting needed vaccines.

The AAP, which announced its stance to members in June, reiterates its position and provides three commentaries on the issue in the January print edition of Pediatrics, published online Dec. 17.

Thimerosal contains a form of mercury called ethyl mercury. For years it was used in certain vaccines, but U.S. health officials decided in 1999 that thimerosal should be removed from most vaccines given to young children. (The exception is some flu vaccines.)

That was done as a precautionary measure until researchers could learn more about how the ethyl mercury in thimerosal might affect children's development. Ethyl mercury is different from methyl mercury -- the form found in the environment that can harm young children's developing brains.

"Back then, there were no guidelines on ethyl mercury exposure," said Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia.

"So the thinking at the time was, let's exercise the precautionary principle," Offit said.

Numerous international studies since have uncovered no evidence of harm -- including no link between thimerosal and autism, which had been a concern. The U.S. Centers for Disease Control and Prevention says there is "no convincing evidence of harm caused by the low doses of thimerosal in vaccines."

Given that, banning thimerosal globally could erode the already low vaccination rates in developing countries for no potential benefit, said Dr. Walter Orenstein, a member of the AAP Committee on Infectious Diseases and co-author of one of the three new commentaries.

Developing nations rely on vaccines containing thimerosal, Orenstein said. The preservative is used in vials that contain more than one dose of a vaccine, to prevent contamination, which can happen when a syringe needle is inserted into the vial.

Rich countries such as the United States can get around the need for thimerosal by using single-dose vials. But for poor countries, multi-dose vials make vaccination programs more feasible, Orenstein explained.

Switching to single-dose vials would pose practical problems. For instance, local clinics would need much more refrigeration space to house the same number of vaccine doses, Orenstein said.

Offit agreed. "These countries have limited resources," he said. "Children there are already under-vaccinated. If there's a ban, we'll be under-vaccinating them even more."

The AAP and WHO statements are in response to an international treaty being hammered out by the U.N. Environmental Program. The treaty would try to reduce mercury exposure from a variety of sources -- from consumer products to medical equipment to emissions from coal-fired power plants. The UN is considering including thimerosal on that list.

Dr. Michael Smith, a pediatric infectious disease specialist at the University of Louisville in Kentucky, said he agreed with the AAP stance.

"Since 1999, there have been multiple studies that have not found any link between thimerosal and autism, but at the time this information did not exist," Smith said, so the United States decided to be cautious and remove the preservative.

"It turns out this was not necessary," Smith said.

Offit used stronger terms. "We made a mistake in our country," he said. "To make the same mistake now, with the information we have now, it could result in thousands of deaths."

Not everyone is convinced thimerosal is risk-free, though. Among them is Barbara Loe Fisher, president of the National Vaccine Information Center, a Virginia-based advocacy group

"If unused vials of thimerosal-containing vaccines must be disposed of as hazardous waste because of the mercury content, then why is the American Academy of Pediatrics strongly advocating that thimerosal-containing vaccines continue to be injected into children's bodies?" Fisher asked.

She added that U.S. health officials have not changed their stance regarding children here. "The Environmental Protection Agency and the Food and Drug Administration have not rescinded the 1999 directive to the pharmaceutical industry to take thimerosal out of childhood vaccines," she said.

And it's unlikely that the agencies will, Orenstein said. "I don't see any reason that the U.S. would add thimerosal back into childhood vaccines," he said.

So whatever happens with the U.N. treaty, it probably won't affect routine childhood vaccinations in the United States. But, Orenstein noted, a global ban on thimerosal could conceivably be a problem in the event of an emergency, such as a flu pandemic. If Americans were suddenly clamoring for the flu vaccine, multi-dose vials would be the best way to get it out quickly.

More information

The U.S. Centers for Disease Control and Prevention has more about thimerosal in vaccines.