Immunization Safety Review Thimerosal-Containing Vaccines and Neurodevelopmental Disorders (2001) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-18
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determined that under the recommended childhood immunization schedule, infants might be exposed to cumulative doses of ethylmercury that exceed some federal safety guidelines established for exposure to methylmercury, another form of organic mercury (Ball et al., 2001~. The methylmercury exposure limits calculated by these agencies are not limits above which injury is certain to occur.
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Speech or language delay refers to several disorders characterized by significant impairments in vocabulary and difficulty in forming sentences, comprehending words or sentences, or forming age-appropriate speech sounds. The Institute of Medicine's Immunization Safety Review Committee is responsible for examining a broad variety of vaccine-safety concerns.
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The hypothesis that thimerosal exposure through the recommended childhood immunization schedule has caused neurodevelopmental disorders is not supported by clinical or experimental evidence because: . low-dose thimerosal exposure in humans has not been demonstrated to be associated with effects on the nervous system,
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some children who received the maximum number of thimerosal-containing vaccines on the recommended childhood immunization schedule had exposures to ethylmercury that exceeded some estimated limits of exposure based on federal guidelines for methylmercury intake, and . some children could be particularly vulnerable or susceptible to mercury exposures due to genetic or other differences.
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The only other epidemiological analysis presented to the committee is an unpublished ecological analysis of rates of autism during the period of increased exposures to thimerosal through the recommended childhood immunization schedule (Blaxill, 2001~. Absent other controlled epidemiological analyses, ecological data are usually noncontributory to causality assessments.
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SIGNIFICANCE ASSESSMENT Even though vaccines on the recommended childhood immunization schedule that are manufactured today and given to children six years of age or younger do not contain thimerosal as a preservative, the hypothesis that exposure to thimerosal-containing vaccines may be associated with neurodevelopmental disorders remains of public health significance. First, the diseases against which thimerosal-containing vaccines were directed diphtheria, tetanus, pertussis, Haemophilus influence type b (Hib)
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The presence of mercury in some vaccines can raise doubts about the entire system of ensuring vaccine safety, and late recognition of the potential risk of thimerosal in vaccines may contribute to a perception among some that careful attention to vaccine components has been lacking. There are no data that elucidate how much, if any, mercury exposure from all sources contributes to the prevalence of autism, ADHD, or speech or language delay.
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In the United States, thimerosal has been removed from most vaccines and some biological products to which infants, children, and pregnant women are exposed. A1though mercury exposures from currently available thimerosal-containing products may not exceed estimated exposure limits for methylmercury derived from federal guidelines, further action to remove thimerosal from all vaccines and other biological and pharmaceutical products might be warranted to ensure that exposures to thimerosal do not contribute to combined mercury exposures that could exceed guidelines for safe exposure.
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As the immunization schedule becomes more complex, and as vaccine safety concerns continue to emerge, it is likely that the public health community, medical professionals, and vaccine manufacturers will again be faced with the need to consider action regarding vaccine safety in the face of great uncertainty and of theoretical rather than demonstrated risks. The committee recommends that policy analyses be conducted that will inform these discussions in the future.
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In addition, because thimerosal poisonings were associated with adverse renal effects, renal outcomes should also be included in the epidemiological, clinical, and basic science studies of thimerosal exposure recommended in this report. Although there are many challenges that will arise in planning and conducting these studies (e.g., appropriate control group selection, conducting a retrospective assessment of mercury exposure from other sources)
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to treat autistic children, these practitioners report unusual metal profiles in their patients as well as clinical improvement following chelation therapy.
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Given that chelation therapy is not a benign treatment, the committee recommends careful, rigorous, and scientific investigations of chelation when used in children with neurodevelopmental disorders, especially autism. Although studies of chelation would not be able to link excreted metal specifically with vaccine exposure, and therefore would not contribute to causality assessments, it is important to pursue these uncontrolled clinical observations in order to establish an evidence base for appropriate therapeutic uses of chelation.
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The limited and unpublished epidemiological data constitute weak and inconclusive evidence regarding causality. However, because thimerosal remains in vaccines in other countries and in biological and pharmaceutical products in the United States, and because it is important to restore and maintain trust in vaccines, the committee believes that continued public health attention must be paid to this issue in the form of policy review and analysis, public health and biomedical research, and improved communication strategies.
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MMWR Morb Mortal chicly Rep 48~26~:563-565.
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Vaccine Safety Datalink (VSD) Screening Study and Follow-Up Analysis with Harvard Pilgrim Data: Cambridge, Massachusetts.
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TABLE 1 Summary Categories and Levels of Evidence Regarding Causality IOM, IOM, 1994a LevelofEvidence 1991a 1 No evi- No evidence No case reports or epidemiological dence bearing on a studies identified. bearing on a causal relation causal relation 2 Evidence The evidence One or more case reports or epide insufficient is inadequate miological studies were located, but to indicate a to accept or the evidence for the causal relation causal reject a causal neither outweighs nor is outweighed relation relation by the evidence against a causal relation.
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