Did the Thimerosal Used as a Preservative in Vaccines Cause Autism?

High levels of mercury are known to damage the nervous system and kidneys. In addition, studies in the Faroe Islands, the Seychelles, and Iraq showed that fetuses might be harmed when pregnant women ingest large quantities of mercury. For these reasons, the FDA Modernization Act of 1997 required the FDA to compile a list of drugs and foods that contain mercury. At that time (and really since the beginning of the modern vaccine era), some vaccines used thimerosal, which contains ethylmercury as a preservative, so these vaccines were included in the FDA list. Preservatives were necessary in multidose vials to prevent contamination with bacteria or fungi.

The cumulative level of mercury represented by the routine vaccine schedule for infants was within the acceptable range published by the FDA, the Agency for Toxic Substance and Disease Registry, and the WHO. However, it slightly exceeded the level considered to be safe by the Environmental Protection Agency (EPA). To determine safe levels of mercury, the EPA evaluated a study performed in Iraq where pregnant women were accidentally exposed to large quantities of methylmercury (a more toxic organic compound than ethylmercury) that had been used to disinfect grain. The EPA then estimated the lowest dose of mercury that was found to cause neurodevelopmental delay in infants as a result of fetal exposure. From this, the lowest dose of methylmercury that could possibly harm an unborn child was calculated and then divided by ten, yielding a very conservative estimate of the lowest acceptable dose of mercury.

There are many problems with using the study in Iraq to determine levels of thimerosal in vaccines that would be safe in children. Among them is the fact that the mercury contained in thimerosal is in the form of ethylmercury, which behaves in the body much differently than methylmercury. In addition, vaccines are administered to children after, not before, they are born, when the nervous system is more mature and, therefore, much less likely to be susceptible to harmful effects. Nevertheless, the Public Health Service and the AAP issued a joint statement on July 9, 1999, calling for manufacturers to eliminate thimerosal from vaccines as a precautionary measure, stating the following: “The current levels of thimerosal will not hurt children, but reducing those levels will make safe vaccines even safer.”

One might wonder how safe vaccines can be made safer, particularly by removing a component that had not been shown to be harmful in the first place. The consequences of this statement ranged from confusion on the part of providers, dismantling of the machinery that had been put in place to deliver the birth dose of HepB, failure to give HepB to many high-risk infants, an onslaught of litigation, and a loss of public confidence. The effects were still evident in 2010, with a barrage of celebrities on television talk shows claiming that thimerosal is responsible for the “epidemic” of autism, op-ed pieces in newspapers, and nearly 5000 thimerosal injury claims pending under the National Vaccine Injury Compensation Program.

The issue reached a penultimate climax in early 2008, when the special federal court for the VICP ruled that multiple vaccinations received in a single day had aggravated an underlying mitochondrial disorder in a child, ultimately manifesting as regressive encephalopathy with features of autistic spectrum disorder. This reignited the controversy, even though the ruling was strictly applicable only to this case of a child with a previously undiagnosed and very rare metabolic defect.

The climax itself occurred in early 2009. Because of the overwhelming burden of petitions before the VICP claiming that vaccines cause autism, the vaccine court asked the petitioners to put forward three test cases for each of three theories, in what became known as the Omnibus Autism Proceedings. The theories were that 1) MMR and thimerosal-containing vaccines combine to cause autism; 2) thimerosal-containing vaccines alone cause autism; and 3) MMR alone causes autism (this theory was subsequently dropped because the evidence was presented as part of the first theory).

On February 12, 2009, the court handed down its decision in the first theory. A typical case that is heard in the vaccine court involves a review of 10 medical articles and the testimony of 2 to 6 expert witnesses. The proceedings for the first theory generated 5,000 pages of transcript, more than 700 pages of post-hearing briefs, 50 expert reports, and involved the testimony of 28 experts and a review of 939 medical articles. The first case (Cedillo vs Secretary of Health and Human Services, No. 98-916V) was heard in June 2007, the second (Hazlehurst vs Secretary of Health and Human Services, No. 03-654V) in October 2007, and the third (Snyder vs Secretary of Health and Human Services, No. 01-162V) in November 2007.

The petitioners only had to prove their case by a “preponderance of the evidence.” Despite this, their claims for compensation were denied, and the judges did not mince words. In essence, they stated unequivocally in each case that the theory was incorrect and that the evidence presented was marginal, paling in comparison to the validated, reproducible, rigorous scientific evidence that had been generated over the years in many parts of the world. Each of the decisions was appealed, and each appeal was denied.

On March 12, 2010, the decisions for the three test cases of the second theory were handed down (Dwyer vs Secretary of Health and Human Services, No. 03-1202V; King vs Secretary of Health and Human Services, No. 03-584V; Mead vs Secretary of Health and Human Services, No. 03-215V). Once again, the court unequivocally rejected the petitioners claims that thimerosal caused autism in these children. And, once again, the judges did not mince words. Special Master Vowell, for example, wrote in the Dwyer case, “The witnesses setting forth this improbable sequence of cause and effect were outclassed in every respect by the impressive assembly of true experts in their respective fields who testified on behalf of respondent.”

Thimerosal in vaccines did not cause autism or any other neurodevelopmental problem. Here is a summary of the evidence:

  • Toxic levels of mercury are not found in infants receiving thimerosal-containing vaccines—In a pilot study published in 2002, 40 full-term infants ≤6 months of age were given vaccines containing thimerosal and 21 were given thimerosal-free vaccines. No infants had blood mercury concentrations exceeding 29 parts per billion, the level thought to be safe in cord blood. Stool concentrations were high, suggesting elimination through the gastrointestinal tract. A follow-up study looking at 216 infants was published in 2008. The blood half-life of mercury after administration of thimerosal containing vaccines was 3.7 days. The highest levels of mercury were seen in the first 24 hours after vaccination, and all were ≤8ng/mL (some infants received as much as 57.5mcg of mercury at one time through the vaccinations). Inorganic mercury was detected in stools within days. The results suggest rapid elimination of ethylmercury from the body after vaccination with thimerosal-containing vaccines.
  • Autism rates continued to increase after thimerosal was removed from vaccines—Whether one looks in Denmark and Sweden, where thimerosal was removed from vaccines in 1992, or Canada, where no vaccines contained thimerosal after 1996, the data are remarkably consistent—continued increases in reported cases of autism. A study from California published in 2008 showed no decrease in autism prevalence by age or birth cohort several years after the last lots of thimerosal-containing vaccines would have expired. As discussed earlier, epidemiologists do not uniformly agree that there is a true increase in the incidence of autism. However, there are many cases being diagnosed, and while the causes remain elusive, the ecologic data argue very strongly against thimerosal being one of them.
  • Cohort studies provide strong evidence against an association—A retrospective cohort study involving 109,863 children in the United Kingdom born between 1988 and 1997 found no convincing associations between thimerosal exposure and developmental disorders. The outcomes of autism or autistic-spectrum disorder were no more common in 1,660,159 person-years of exposure to thimerosal than they were in 1,220,006 person-years of nonexposure. The study further showed no evidence of a dose-response relationship between cumulative amounts of ethylmercury exposure and autism. A prospective cohort study in the United Kingdom (children were enrolled at birth and behavioral data were collected regularly thereafter) involving over 14,000 children showed that poor prosocial behavior at 47 months of age was correlated with ethylmercury exposure by 3 months of age. However, the same study showed that ethylmercury exposure was associated with better outcomes in eight other areas, including conduct, fine motor development, reported tics, and the need for special education. A 2-phase retrospective cohort study in the United States involving over 100,000 children also found no consistent associations with neurodevelopmental outcomes. Finally, in a 2007 study, 1047 children were given a 3-hour assessment involving 42 different neuropsychologic tests; no consistent associations were seen between earlier exposure to thimerosal-containing vaccines and the test results. 

Authoritative bodies are virtually unanimous in rejecting the purported link between thimerosal and autism. As of 2010, the only routine childhood vaccines that still contain thimerosal as a preservative are some brands of IIV (although preservative-free formulations for children are available). Thimerosal activists argue, however, that vaccines labeled preservative-free are still not safe because they contain trace amounts of thimerosal, levels that are essentially below the limit of detection. Since a product cannot be labeled thimerosal-free if thimerosal is used anywhere in the manufacturing process, companies have either dropped out of the market or scrambled to revise their protocols and package their products in single-dose vials or prefilled syringes. The public has paid the price in dollars as well as in vaccine availability. Some autism advocacy groups have urged that we turn our attention away from vaccines and towards more promising lines of research. In fact, there is a growing body of evidence that genetic abnormalities play a role in the development of autism.

—Marshall, Gary S. "Addressing Concerns About Vaccines." The Vaccine Handbook: A Practical Guide for Clinicians. 3rd ed. New York: Professional Communications, Inc., 2010. 219-229. Print.

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