An analysis of blood-mercury levels in infants immunized with vaccines containing a preservative with traces of the element showed the mercury was cleared within a month and caused no developmental problems, scientists reported yesterday.

Thimerosal a vaccine preservative that has been in use globally for more than a half-century, had been at the core of a debate in this country about vaccine safety.

In 1999, thimerosal, which contains ethyl-mercury, was banned from vaccine production in the United States. It was removed as a precautionary measure to cut down on exposure to mercury from all sources. However, it is still used in the manufacture of vaccines abroad.

Dr. Michael Pichichero, a pediatrician and immunologist at the University of Rochester, said he and his team studied 61 infants between 1999 and 2000 who had received thimerosal-containing vaccines. While traces of mercury showed up in initial blood tests, the element was completely cleared within 30 days.

"These results are very encouraging," Pichichero said. He added that the investigation confirms a previous medical hypothesis that the amount of mercury in the vaccines was far too low to cause harm.

Thimerosal was added to multi-dose vials of a variety of vaccines, explained Dr. John Treanor, a University of Rochester vaccine expert and member of the research team. The preservative helped maintain the potency of the doses by preventing contamination from bacteria. "Contamination can be a problem because someone uses these vials over and over," Treanor said.

Results of the study, which focused on children between the ages of two months and six months, is reported in the British medical journal The Lancet.

The report was previewed by experts at the World Health Organization, which issued a statement endorsing the continued use of thimerosal in vaccines elsewhere in the world. Vaccines can be difficult to maintain in some regions of the globe where refrigeration is undependable. WHO officials underscored that while the cost of switching to single-dose vaccines does not pose a problem for a country as wealthy as the United States, poorer nations cannot afford the production costs.