Mercury and Vaccines (Thimerosal) : Part 2

by CDC

(Page 2 of 3)

What research is being conducted by the Federal Government regarding the safety of vaccines containing thimerosal?

There is no evidence to suggest that thimerosal in vaccines causes any health problems in children and adults beyond local hypersensitivity reactions (like redness and swelling at the injection site). However, efforts to remove thimerosal from the U.S. vaccine supply have been accompanied by research investigations to better assess the potential health effects of exposure to thimerosal containing vaccines:

The National Institute of Allergy and Infectious Diseases at the National Institutes of Health funds thimerosal research that focuses on better understanding of what happens to thimerosal once it is introduced into the body and how this compares to current knowledge of methyl mercury pathways.

A recent study sponsored by the NIAID and conducted at the University of Rochester assessed mercury levels in 40 infants who received vaccines containing thimerosal and 21 infants who received thimerosal-free vaccines. The scientists measured the level of mercury in the infants' blood, urine and stool up to 28 days after vaccination. They found that 1) infants who were given vaccines with thimerosal had levels of mercury well below the safe level of 29 nmol/L (this level is set ten times lower than the level at which mercury begins to cause neurological problems) and 2) the body seems to be able to get rid of thimerosal (ethyl mercury) via the gastrointestinal tract (stools) much quicker than it gets rid of methyl mercury.

NIAID and the National Institutes of Environmental Health Sciences are also funding studies comparing the pharmacokinetics and tissue distribution of thimerosal, ethyl mercury, and methyl mercury in non-human primates. Pharmacokinetics is the study of how an agent is absorbed, distributed, metabolized (broken down), and excreted.

The CDC's Center for Environmental Health and the National Center for Health Statistics are doing a study looking at all mercury exposures and working with the National Health and Nutrition Examination Survey. NHANES 4 will collect samples of blood, hair and urine from all women of reproductive age and children under 5 in order to assess mercury levels in the body from all sources of mercury a person can be exposed to in the environment. Findings of a study conducted using NHANES 3 data to check blood and hair mercury levels suggest that the mercury levels in young children and in women of childbearing age are generally below the level considered hazardous.

Does thimerosal cause autism?

There is no conclusive evidence that any vaccine or vaccine additive increases the risk of developing autism or any other behavior disorder. Rather, evidence is accumulating of lack of any harm resulting from exposure to vaccine containing-thimerosal as a preservative. In a 2004 report, the Institute of Medicine (IOM) concluded that there is no association between autism and vaccines that contain thimerosal as a preservative. Nonetheless, given the level of concern among parents and others regarding vaccines and autism, the CDC is committed to investigating this issue to the fullest extent possible, using the best scientific methods available.

What about a 2003 study that claimed to find a relationship between thimerosal in vaccines and autism, speech disorders, and heart disease?

In 2003, Geier and Geier reported conducting two analyses to test whether thimerosal in vaccines is associated with autism, speech disorders, and heart disease. The researchers inadequately described the methods they used, making it impossible to determine exactly what was done and how the results should be interpreted. In the first analysis, the researchers reviewed Vaccine Adverse Event Reporting System (VAERS) reports involving autism, speech disorders, and heart disease. They state that they compared VAERS reports involving thimerosal-containing DTP and DTaP vaccines with those involving thimerosal-free DTaP vaccines. There are a number of weaknesses in this analysis, including an apparent misunderstanding among the authors regarding VAERS reporting requirements. VAERS is a passive surveillance system for reporting possible vaccine adverse events that depends on health care providers, patients, and/or others to file reports. Health effects reported to VAERS as following vaccination may be true adverse events, coincidental occurrences, or mistakes in filing. Because of this, VAERS has certain weaknesses that limit the system, including incomplete reporting, lack of verification of diagnoses, and lack of data on those who were vaccinated and did not report problems. VAERS data are useful for "hypothesis generation" (raising questions), but should not be used for research aimed at determining whether vaccines cause certain health problems as done by Geier and Geier. Moreover, children who could have received thimerosal-free DTaP vaccine were less likely to have autism or speech disorders diagnosed because they were younger than the children in the thimerosal-containing vaccines group. In the heart disease evaluation, the authors examined reports coded as "heart arrest," these cases in VAERS are completely different from the coronary heart disease cases in the studies that the authors cite to support a possible association between mercury and heart disease. It is also unclear how the researchers calculated levels of mercury exposure, because their calculations are inconsistent with the known levels of mercury that were in thimerosal-containing DTP or DTaP vaccines.

About the Author

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The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.