Does thimerosal in vaccines pose a risk to infants?
When pregnant women eat foods or take medicines that contain mercury, the mercury can be transferred to the developing fetus through the placenta. Infants can be exposed to mercury through foods, including breast milk, or medicines.
Developing fetuses and young children are believed to be more susceptible to mercury exposure than adults because mercury can interfere with the developing nervous system.
Guidelines for safe exposure to methylmercury, based on the analysis of cases where people were accidentally exposed to toxic levels of mercury, have been developed by three federal agencies1. Although the three agencies’ guidelines are each slightly different, each leaves a large margin for safety, and exposure to amounts that exceed these guidelines does not mean that the individual has been exposed to toxic levels of mercury.
Additionally, it should be noted that, some studies show that ethylmercury (the kind to which thimerosal is metabolized) may be less toxic than methylmercury (the kind that was used in establishing the safety guidelines). However, because little information about ethylmercury has been available until recently, guidelines for thimerosal safety have been based on methylmercury guidelines.
As part of the Food and Drug Administration (FDA) Modernization Act of 1997, the FDA began compiling a list of the amount and type of mercury in drugs and foods.
Notably, since the last formal FDA review of thimerosal use in biologics in 1976, two important things have changed regarding vaccines: there have been advances in the understanding of the human health effects of low-level exposure to mercury, and there has been an increase in the number of vaccines recommended for routine use in children2.
In their recent review, the FDA found that, depending on which formulation an infant received for each of his or her recommended vaccines, the infant could potentially be exposed on an immunization day to total levels of mercury that would exceed the Environmental Protection Agency (EPA) guideline of 0.1 micrograms of methylmercury per kilogram of infant body weight per day. (See also FDA’s response to a petiton to suspend and revoke all vaccines containing thimerosal for which there is a thimerosal-free replacement available).
This should have posed no risk to the child because the guidelines were established based on exposure to this amount of mercury every day. (See the National Academy of Science’s National Research Council July 2000 review of the EPA guideline.) Nevertheless, this finding led to the request for removal of thimerosal from vaccines and the temporary suspension of the birth dose of hepatitis B vaccine until formulations of the vaccine became available that did not contain thimerosal as a preservative.
Many questions are being asked about the potential effect of thimerosal on the developing fetus and infant, in particular on the developing nervous system. To begin, how is thimerosal processed in the bodies of infants?
In one study, scientists at the University of Rochester Medical Center tested the blood levels of mercury in 16 full-term infants shortly after the children had received recommended vaccines that contained thimerosal. They found that “none of the blood mercury levels observed in the studied infants exceeded the most recently revised lowest level of maternal blood mercury considered to represent a potentially significant exposure to the developing fetus.”
More research is planned to evaluate if the thimerosal in vaccines poses a risk to children. The study also suggested that unlike the toxin methylmercury, thimerosal is eliminated through stools within two weeks.
Developing fetuses and young children are believed to be more susceptible to mercury exposure than adults because mercury can interfere with the developing nervous system.
Guidelines for safe exposure to methylmercury, based on the analysis of cases where people were accidentally exposed to toxic levels of mercury, have been developed by three federal agencies1. Although the three agencies’ guidelines are each slightly different, each leaves a large margin for safety, and exposure to amounts that exceed these guidelines does not mean that the individual has been exposed to toxic levels of mercury.
Additionally, it should be noted that, some studies show that ethylmercury (the kind to which thimerosal is metabolized) may be less toxic than methylmercury (the kind that was used in establishing the safety guidelines). However, because little information about ethylmercury has been available until recently, guidelines for thimerosal safety have been based on methylmercury guidelines.
As part of the Food and Drug Administration (FDA) Modernization Act of 1997, the FDA began compiling a list of the amount and type of mercury in drugs and foods.
Notably, since the last formal FDA review of thimerosal use in biologics in 1976, two important things have changed regarding vaccines: there have been advances in the understanding of the human health effects of low-level exposure to mercury, and there has been an increase in the number of vaccines recommended for routine use in children2.
In their recent review, the FDA found that, depending on which formulation an infant received for each of his or her recommended vaccines, the infant could potentially be exposed on an immunization day to total levels of mercury that would exceed the Environmental Protection Agency (EPA) guideline of 0.1 micrograms of methylmercury per kilogram of infant body weight per day. (See also FDA’s response to a petiton to suspend and revoke all vaccines containing thimerosal for which there is a thimerosal-free replacement available).
This should have posed no risk to the child because the guidelines were established based on exposure to this amount of mercury every day. (See the National Academy of Science’s National Research Council July 2000 review of the EPA guideline.) Nevertheless, this finding led to the request for removal of thimerosal from vaccines and the temporary suspension of the birth dose of hepatitis B vaccine until formulations of the vaccine became available that did not contain thimerosal as a preservative.
Many questions are being asked about the potential effect of thimerosal on the developing fetus and infant, in particular on the developing nervous system. To begin, how is thimerosal processed in the bodies of infants?
In one study, scientists at the University of Rochester Medical Center tested the blood levels of mercury in 16 full-term infants shortly after the children had received recommended vaccines that contained thimerosal. They found that “none of the blood mercury levels observed in the studied infants exceeded the most recently revised lowest level of maternal blood mercury considered to represent a potentially significant exposure to the developing fetus.”
More research is planned to evaluate if the thimerosal in vaccines poses a risk to children. The study also suggested that unlike the toxin methylmercury, thimerosal is eliminated through stools within two weeks.
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