Jan. 31 (Bloomberg) -- GlaxoSmithKline Plc, Europe's largest drugmaker, plans to begin selling a new formula of its Pediarix vaccine for children that doesn't contain preservatives that some parents fear may trigger autism.

The U.S. Food and Drug Administration approved Glaxo's request to change labeling for the new shot to say that it is free of preservatives, according to a letter posted today on the agency's Web site. Some parents and advocacy groups have raised concerns that thimerosal, a preservative containing mercury, may cause some children who are vaccinated to develop autism.

The FDA has been working with vaccine makers since at least 1999 to reduce or eliminate thimerosal, although health officials maintain that a link to autism hasn't been scientifically proven. Autism is an umbrella term for a range of developmental and communications disorders that affect as many as six of every 1,000 children in North America.

Glaxo removed trace amounts of the preservative in its new Pediarix formula and plans to phase in the new products ``in the near future,'' said Jennifer Armstrong, a spokeswoman for the London-based company.

``Our re-formulation has been made in response to public concern,'' Armstrong said today in a telephone interview. ``It's a very minor change.''

Pediarix is the only FDA-approved combination vaccine to treat five childhood diseases. The vaccine is given in three doses and protects infants from diphtheria, tetanus, polio, whooping cough and hepatitis B. About 21 million doses of Pediarix have been given in the U.S. since 2003.

Thimerosal has been used in drug products since the 1930s to prevent the growth of bacteria and fungi, according to the FDA's Web site.

To contact the reporter on this story: Catherine Larkin in Washington at clarkin4@bloomberg.net.

Last Updated: January 31, 2007 16:43 EST