A federally funded study offers reassurance to those who are concerned about the health risks of vaccines containing the preservative thimerosal. Researchers found that blood mercury levels in vaccinated infants were well below those considered safe and that mercury was eliminated from the body much faster than expected.

The findings suggest that government health officials may have acted prematurely in recommending the removal of thimerosal from vaccines in the summer of 1999. The move came in response to public fears that the mercury in the preservative may be linked to an increase in autism, asthma, and several other childhood diseases. Vaccines containing thimerosal are no longer given to children in the U.S., but the preservative is still widely used in vaccines given to children in other countries. Removal of the preservative has reportedly raised costs of producing, storing, and shipping vaccines.

"Our most important finding was that not a single child had a blood mercury level which exceeded the most stringent safety standards -- those set by the Environmental Protection Agency [EPA]," lead researcher Michael E. Pichichero, MD, said. "We also showed that the mercury in vaccines is eliminated from the body six to seven times faster than has been predicted."

In the study, reported in The Lancet, Pichichero and colleagues from New York''s University of Rochester Medical Center examined mercury levels in the blood, urine, and stool of 40 infants given vaccinations containing thimerosal and 21 infants who received thimerosal-free vaccines. All infants received diphtheria-tetanus-acellular pertussis (DtaP) and hepatitis B vaccines, and some were also given an Haemophilus influenzae type B vaccine. These immunizations are typically given to children at the ages of 2 months, 4 months, and 6 months.

All of the children were found to have mercury levels below the EPA''s public safety limit of 5.8 nanograms per milliliter (ng/mL). Most had blood mercury levels ranging from 1 to 2 ng/mL, and the highest level found was 4.11 ng/mL.

The children eliminated half of the vaccine-related mercury in their stools within six to seven days of inoculation, instead of the 45 days that had been predicted by previous studies. This suggests that typical vaccination schedules are safe, Pichichero says. "Vaccines are usually given to infants two months apart, and our findings suggest that by 60 days there is virtually no mercury left in the body," he says. "So there is no cumulative effect."

Immunization expert Neal Halsey, MD, who has been outspoken in calling for increased vaccine safety research, calls the National Institute of Allergy and Infectious Diseases-funded study a good start.

"This is a good study, and it helps us to understand the metabolism of ethyl mercury associated with thimerosal," says Halsey, director of the Institute for Vaccine Safety at Baltimore''s Johns Hopkins Bloomberg School of Public Health. "But one thing we don''t know from this study is what the peak levels of mercury are shortly after vaccination."

The University of Rochester researchers hope to address this question in a similar study of 200 children now under way in Argentina. But Halsey says the definitive answer to the safety question is at least several years away. That is when results are expected from a separate government follow-up study of children exposed to differing levels of thimerosal.

"Everyone has a small amount of mercury in their body from different exposures," he said. "The question is what level is associated with harm in young children."

Article courtesy: The Lancet.