Blood ethyl mercury levels peaked at no higher than 8 ng/mL shortly after vaccination and then returned to prevaccination levels within weeks, reported Michael E. Pichichero, M.D., of the University of Rochester here, and colleagues in the February issue of Pediatrics.
Half of the ethyl mercury was cleared within 3.7 days, about 10 times faster than had been shown for adults consuming methyl mercury from food, they said.
The findings represent still another blow to concerns that thimerosal could cause autism or other pervasive developmental disorders, Dr. Pichichero and colleagues said. The data should be reassuring to parents whose children were vaccinated before 2001 when thimerosal was largely eliminated from vaccines in the United States and Europe, they added.
However, the American Academy of Pediatrics expressed alarm that parents might be swayed nonetheless by the television show "Eli Stone."
Previews of the premier episode set to air Thursday on ABC present a mother's argument that thimerosal caused her child's autism. Media reports suggest the episode will include a lawyer supporting her case in court against a pharmaceutical company.
The AAP called on ABC to cancel the episode. It said the show had the mother winning her case.
The message could "undermine the years of efforts by the AAP and public health community to persuade parents to vaccinate and protect their children," according to a statement from AAP president RenÃ©e R. Jenkins, M.D., of Howard University and George Washington University.
She said a report linking the measles vaccine to autism in England prompted a decline in vaccination leading to the death of several children in the worst outbreak of measles there in two decades.
"ABC will bear responsibility for the needless suffering and potential deaths of children from parents' decisions not to immunize based on the content of the episode," Dr. Jenkins said.
Preview of â€œEli Stoneâ€
Thimerosal was widely used as an antibacterial agent in multidose vaccine vials before the Environmental Protection Agency announced in 1999 that the cumulative dose of ethyl mercury from thimerosal-containing vaccines in childhood might exceed safe limits based on estimates from oral intake of methyl mercury.
However, it continues to be used elsewhere with the support of the World Health Organization, Dr. Pichichero and colleagues said.
They studied mercury excretion among 216 infants born at a hospital in Buenos Aires, where thimerosal is still routinely used in vaccines. In the prospective observational study, children had their blood mercury levels tested both before and after receiving routine vaccinations at their newborn, two-, or six-month checkup. Stool and urine samples were likewise collected.
Newborns received vaccines totaling 12.5 to 32.5 Âµg of mercury via intramuscular vaccination. The two-month-old and six-month-old groups received a further 37.5 to 57.5 Âµg of mercury. The cumulative dose of ethyl mercury was 112.5 to 162.5 Âµg.
Blood mercury levels were relatively low but peaked soon after vaccination when tested at 12 hours in the newborn group and at 24 hours in the two- and six-month-old groups.
The highest level detected was 8.0 ng/mL in a newborn 12 hours after receiving the birth dose of a hepatitis B virus vaccine containing 32.5 Âµg of mercury.
Blood mercury levels largely returned to baseline within 11 days and appeared to exit the body in the stool rather than the urine.
The mercury did not appear to cause renal toxicity because no clinically significant elevations of urine gamma-glutamyl transpeptidase levels were seen.
The half-life of blood ethyl mercury was substantially lower than the 44 days previously reported for methyl mercury. The researchers estimated the half-life after vaccination to be 3.7 days for newborns, 2.0 days for two-month-olds, and 2.2 days for six-month-olds.
Dr. Pichichero said these findings suggest the assumptions made in the late 1990s that ethyl mercury would behave the same as methyl mercury were incorrect.
The researchers noted that their measurements may have overestimated the amount of blood mercury contributed by ethyl mercury from vaccines since both methyl and ethyl mercury were present in the blood of some children.
And although the study was limited by lack of 24-hour sample collection and could not determine the ultimate fate of mercury after it left the blood, Dr. Pichichero expressed confidence in the low toxicity risk found in the study.
While "the importance of blood levels of ethyl mercury for assessing toxicity is unknown," the researchers said, " the low levels of mercury detected in this study suggests relatively low risk for toxicity from this exposure."
PediatricsSource Reference: Pichichero ME, et al "Mercury levels in newborns and infants after receipt of thimerosal-containing vaccines" Pediatrics 2008; 121: e208 - e214.