The first detailed analysis of blood mercury levels in infants who received vaccines containing the preservative thimerosal indicates that blood levels of mercury in children are comfortably below current safety limits. The study of 61 children by physicians and scientists at the University of Rochester Medical Center, also found that the form of mercury in vaccines is eliminated from the blood much more quickly than scientists had predicted.

"The results are very reassuring," says pediatrician Michael E. Pichichero, the lead investigator of the study and professor of microbiology and immunology, pediatrics, and medicine. "The amount of mercury is well below all established safety levels."

The issue is at the core of a national debate over the safety of vaccines. While some parents and politicians have asserted that the minuscule amounts of mercury used in vaccines could be responsible for a range of disorders including autism in some children, no scientific study has found a link. The current study adds evidence to the argument by most pediatricians and public health officials that vaccines are safe.

During the 1990s the number of vaccines given to infants increased markedly, with the addition of immunizations against diseases like hepatitis B and meningitis. Though each vaccine contained only a small amount of thimerosal and a minute amount of mercury, some became concerned that perhaps the cumulative amounts might harm children.

In 1999, the American Academy of Pediatrics and public health officials urged vaccine manufacturers to remove thimerosal from vaccines administered in the United States. The compound has since been removed from nearly all vaccines given to U.S. children, though there is no scientific evidence that the compound has harmed children. The preservative is still used widely in other countries to make vaccines available to millions of children at a lower cost.

In response to the debate, the National Institute of Allergy and Infectious Diseases (NIAID) asked vaccine researchers at the University of Rochester Medical Center to investigate. With funding from NIAID, the Rochester team measured mercury concentrations in urine, blood, and stools of 61 infants -- 40 received vaccines containing thimerosal, and 21 received thimerosal-free vaccines. All the children in the study had received diphtheria-tetanus-acellular pertussis vaccine and hepatitis B vaccine, and some also received Haemophilus influenzae type B vaccine. The immunizations are typically given to children at the ages of two months, four months, and six months.

Most of the children in the study had blood mercury levels of 1 or 2 nanograms per milliliter; the highest level, found in one child, was 4.11 ng/ml. By comparison, the most stringent public safety limit, established by the Environmental Protection Agency, is above 5.8 nanograms per milliliter. That number itself is a small fraction of the amount that scientists believe is the level of mercury that would actually harm a child.

The team also found that children eliminate thimerosal mercury from the blood six times faster than predicted from data on methyl mercury, mostly through the stools. The compound's "half life" in the blood is 6 or 7 days, compared to the 45 days that scientists had assumed. Thus, by the time a child receives another round of vaccines containing mercury, virtually all of the compound from the previous doses has been eliminated.

Removing thimerosal from U.S. vaccines has had several effects, Pichichero says. Since vaccines don't last as long without a preservative, the elimination of thimerosal caused some vaccine makers to produce single-dose vials, which are more expensive to produce, store, and ship. For some vaccines, manufacturers use thimerosal throughout the manufacturing process, then remove the compound, which also adds to the cost of the vaccine. Such actions raised the cost of vaccines, making it less likely that they'll be used as widely as possible.

Thimerosal is still part of vaccines widely used in other nations. In October the World Health Organization announced guidelines suggesting that thimerosal-containing vaccines are safe and should continue to be used, a conclusion based partly on Pichichero's findings.

"In countries that are still confronting diseases like whooping cough and tetanus and measles, where millions of children die of the disease, there is no argument. Where people are dying of these diseases, switching to a thimerosal-free vaccine would raise the prices such that millions of children would go unvaccinated.

While mercury is known to be toxic in high amounts, scientists continue to debate the health effects of exposure to very low levels. Everyone on Earth has some mercury in the blood stream -- the chemical is present naturally, from the belching of volcanoes, and is also present in power-plant emissions. Everyone who smokes cigarettes contributes a bit of mercury to the air we breathe. Mercury is found especially in seafood like swordfish and tuna; a tuna sandwich contains much more mercury than a typical vaccine dose.

The University of Rochester team has just begun a similar but larger study -- of about 200 children -- with funding from NIAID, to elaborate on the results of the study published in the Lancet. The work is being conducted with colleagues in Buenos Aires, Argentina, since most vaccines now given to children in the United States no longer contain thimerosal.