Infants excrete the ethyl mercury used in thimerosal-containing vaccines too quickly for the mercury to build up in their systems, according to a recent study.
The findings may help to quell chronic concerns that thimerosal-containing vaccines can increase a child's risk for developmental problems, such as autism. These concerns led to the removal of thimerosal from most vaccines given to children in the United States and Europe as of March 2001, although it remains a component of vaccines used in other countries.
Previous studies have described how the body processes methyl mercury—the type associated with the consumption of fish—but so far, few studies have examined how the body processes ethyl mercury after exposure via intramuscular injection.
In this study, Dr. Michael Pichichero of the University of Rochester (N.Y.), and his colleagues evaluated the ethyl mercury in blood, urine, and stool samples from 216 healthy children prior to vaccination with thimerosal-containing vaccines and again at several points between 12 hours and 30 days after vaccination, depending on the age group.
The study population included 72 newborns, 72 2-month-old infants, and 72 6-month-old infants who were vaccinated at a children's hospital in Argentina between February 2003 and February 2004 (Pediatrics 2008;121:e208–14).
The main hypothesis of the study was that ethyl mercury does not stay long in the bloodstreams of vaccinated infants. Complete pre- and postvaccination blood samples were available from 128 children (59%): 40 newborns, 50 2-month-olds, and 38 6-month-olds.
Overall, the blood mercury levels in all three age groups were highest at the first postvaccination measurements—12 hours after vaccination for newborns and 24 hours after vaccination for the 2-month-olds and 6-month-olds.
In all of the age groups, blood mercury levels quickly dropped and the levels in most of the children had returned to normal by 11 days after vaccination.
The average maximal blood mercury levels following vaccinations for the newborns, 2-month-olds, and 6-month-olds were 5.0 ng/mL, 3.6 ng/mL, and 2.8 ng/mL, respectively.
Even the highest level of mercury was relatively low. The overall highest level detected in the study was 8.0 ng/mL, and it was noted in a newborn 12 hours after a birth dose of a hepatitis B virus (HBV) vaccine that included 32.5 mcg of mercury.
Another important finding was that the prevaccination levels among the 6-month-olds were not higher than the prevaccination levels among the 2-month-olds. This suggests that ethyl mercury does not accumulate in the blood as a result of exposure to thimerosal-containing vaccines, the researchers commented.
“Using a model that accounted for baseline mercury levels, ethyl mercury dosage, and timing of vaccination, we estimated the blood half-life of mercury after administration of thimerosal to be 3.7 days, which did not vary significantly by age group,” the researchers wrote.
“This study is very timely for something that pediatricians are going to be facing,” Dr. Pichichero said in an interview. The ABC-TV network recently aired the debut of a fictional series about a lawyer, “Eli Stone,” that suggested a conspiracy between vaccine companies and physicians to keep an association between vaccines and autism under wraps.
“So every pediatrician in America is going to face parents who see this fictional program,” added Dr. Pichichero. “We are going to have parents wondering whether their doctors are in cahoots with the vaccine companies.”
Dr. Pichichero feels that this study virtually eliminates the argument that mercury can accumulate to unsafe levels in children as a result of standard infant vaccinations. “Our study shows beyond a shadow of a doubt that that is not true given how quickly ethyl mercury disappeared from the blood,” he said.
In fact, inorganic mercury was found in almost all the stool samples in all age groups. Mercury levels in the stool increased shortly after vaccinations in all age groups, but then they began to decline, which suggests that the mercury is leaving the body in the stool, Dr. Pichichero said. By contrast, the urine samples in all age groups showed barely detectable levels of mercury, which suggests that the kidneys are likely not affected, he added.
The results were limited by the combination of methyl and ethyl mercury in the blood samples from some children and by the researchers' inability to identify the exact proportion of mercury excreted through the stool and urine.
However, the short half-life of ethyl mercury in the blood suggests a need to reassess the risk of thimerosal as a preservative, they wrote.
Dr. Pichichero is an unpaid consultant for the World Health Organization on vaccine-related issues, and he has served as a consultant to several vaccine manufacturers including GlaxoSmithKline Biologicals, Sanofi Pasteur, Wyeth Pharmaceuticals, MedImmune, and Merck & Co.