The findings bolster the argument that a mercury-based vaccine preservative doesn't cause autism in children, although it's unclear from the study whether some mercury may linger elsewhere in the body.
The research addresses an unanswered question about the safety of thimerosal, a preservative that has been eliminated from routine U.S. vaccines, and breaks down as ethyl mercury in the body. It is still used in other countries, including Argentina.
Scientists assumed the health risks from ethyl mercury were similar to methyl mercury from eating fish. And in 1999, the federal government and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines to lower exposure to mercury from all sources. The decision was based on what was known about methyl mercury exposure.
The new findings suggest that methyl mercury and ethyl mercury are very different and that the removal of thimerosal from vaccines may have been over-cautious.
"The study supports the decision by the World Health Organization to continue to permit thimerosal to remain in vaccines for the world's children," said study co-author Dr. Michael Pichichero of the University of Rochester in Rochester, N.Y. He said thimerosal vaccines are cheaper to produce and therefore more accessible to much of the world.
In the U.S.-funded study, blood samples were taken from 216 healthy babies before and after they got vaccines containing the preservative thimerosal. Blood levels of mercury were highest shortly after the babies were vaccinated and fell to pre-vaccination levels within a few weeks.
"The amount found in the blood was about one-tenth of that predicted in that the late 1990s and the length of time it stays in the blood is one-tenth of that predicted," Pichichero said.
Mercury levels also were measured in the babies' stools and urine. In the stool samples, the levels were highest after vaccination and also fell, but more slowly than blood levels. There was no significant amount of mercury in the urine.
The authors could not determine what happened to all the mercury after it left the blood. All the infants gave samples twice: before vaccination and at one other time, ranging from 12 hours later to 30 days.
The study will be published in the February issue of the journal Pediatrics. The medical journal released the findings early because of a controversy surrounding a new TV series premiering Thursday, which features a lawyer who argues in court that a flu vaccine made a child autistic.
The journal is published by the American Academy of Pediatrics, which on Monday called on ABC to cancel the first episode of the series, "Eli Stone," saying that it perpetuates the myth that vaccines can cause autism.
Autism is a complex disorder featuring repetitive behaviors and poor social interaction and communication skills. Scientists generally believe that genetics plays a role in causing the disorder; a theory that thimerosal is to blame has been repeatedly discounted in scientific studies.
Pichichero said he has received research grants and served as a consultant to several vaccine makers, but said there was no industry involvement in the new study. He is an unpaid consultant to the WHO on vaccines.