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Thimerosal-Containing Vaccines and Neurodevelopmental Disorders


Thimerosal Opening Statement
Thimerosal Agenda
ISR Thimerosal
Thimerosal report
HPDP Reports
PARO July 2001 meeting
IOM Reports released in 2001
Immunization Safety Review

Institute of Medicine

This is the html version of the file

This is the pdf version of the file http://www.ama-assn.org/ama/upload/mm/36/iom_thimerosal.pdf

News Conference
Oct. 1, 2001

Opening Statement By Marie C. McCormick


Professor and Chair, Department of Maternal and Child Health Harvard School of Public Health and Chair, Committee on Immunization Safety Review

Good afternoon. On behalf of the Institute of Medicine and the entire committee, I would like to welcome reporters and guests to the release of our report, Thimerosal-Containing Vaccines and Neurodevelopmental Disorders. I am joined by my fellow committee members Joshua Cohen and Bennett Shaywitz.

The genesis of this report was a request from the Centers for Disease Control and Prevention and the National Institutes of Health, both of which recognized the need for an independent group of scientists to address growing concerns about vaccine safety in a timely and objective manner. In response to this request, the Committee on Immunization Safety Review was established in January. The committee comprises 15 members with expertise in a variety of relevant public health and medical disciplines.

During a three-year study period that began this year, the committee is charged with examining nine hypotheses selected by a working group of federal government vaccine researchers and policy-makers. For each hypothesized association between vaccines and adverse health effects, the committee will look at the science behind it, assess its significance in a broader societal context, and suggest appropriate actions to be taken by federal agencies, researchers, and policy-makers. In this study, our committee was asked to consider concerns about the potential relationship between thimerosal -- a mercury-containing preservative used in some vaccines -- and neurodevelopmental disorders such as autism, attention deficit-hyperactivity disorder, or ADHD, and speech or language delay.

First, a bit of context. Childhood immunization is widely regarded as one of the world's most effective tools for preventing millions of cases of disease and death. In the United States alone, child-vaccination programs have eliminated smallpox and polio and made many debilitating and potentially life-threatening infectious diseases, such as diphtheria, pertussis, and measles, extremely uncommon. Along with these benefits, however, have come some concerns about whether children experience any adverse health effects from the vaccines themselves.

Because mercury at high doses is known to pose risks, some parents and researchers are concerned that thimerosal in vaccines puts children at increased risk for developmental disorders such as autism. Preliminary data from a few studies have suggested that thimerosal-containing vaccines could possibly -- very minimally -- affect some measures of normal child development. But the data are inconclusive.

Thimerosal has been used since the 1930s in several childhood and adult vaccines to prevent bacterial contamination in multidose vials. The active ingredient in thimerosal is ethylmercury, a close chemical relative of methylmercury. At high doses, methylmercury and some other forms of the heavy metal are known to damage the nervous system. Ethylmercury has not been well-studied, but the amount in vaccines is small.

Our committee has reviewed the limited body of toxicological, clinical, and epidemiological literature on ethylmercury and the more extensive literature on methylmercury. The committee also held an open scientific workshop in July in Boston. Based on information from these sources, our study has come to the following conclusion: The hypothesis that thimerosal exposure through the recommended childhood immunization schedule causes neurodevelopmental disorders is not supported by clinical or experimental evidence. Existing epidemiological evidence is inadequate to either accept or reject a causal relationship between exposure to thimerosal from vaccines and the neurodevelopmental disorders of autism, ADHD, and speech or language delay. However, there are some indirect associations concerning biological plausibility, which refers to a theoretical but unproven possibility. For example, high-dose thimerosal exposures are associated with neurological damage. There is also toxicological and epidemiological literature suggesting that methylmercury is a toxicant to the developing nervous system. Some children who received the maximum number of thimerosal-containing vaccines on the recommended childhood immunization schedule had exposures to ethylmercury that exceeded some safe exposure guidelines for methylmercury. In addition, some children could be particularly vulnerable or susceptible to mercury exposures because of genetic or other differences.

So, where do we stand today with respect to thimerosal in vaccines? In 1999 the U.S. Public Health Service, the American Academy of Pediatrics, and the American Academy of Family Physicians set a goal of removing thimerosal from vaccines in the United States as soon as possible. This decision was based on several factors. It was viewed as feasible as well as consistent with the public health goal of decreasing mercury exposures in general, as much as possible. Mathematical calculations also suggested that some infants received a total amount of mercury from vaccines that exceeded some federal agency guidelines for safe mercury exposure. It should be noted that exceeding the guidelines for safe exposure does not mean that harm is certain to occur. These guidelines take into account safety factors to protect sensitive populations, such as children. In addition, they were set for methylmercury exposure, which occurs primarily from eating fish, and their relevance to ethylmercury in vaccines is unclear. The policy action of 1999 was a means to decrease cumulative mercury exposures. Neither at that time nor today does any data exist to prove that thimerosal is dangerous at the level present in vaccines.

Our committee, in reaching its two major conclusions, considered the debate in a broad context. The government has a major responsibility to ensure that vaccines are as safe as possible and to understand the science base for safety concerns as well as it can. Although the vaccines on the recommended childhood immunization schedule are no longer made with thimerosal, the preservative remains in some other vaccines and some pharmaceutical products. If thimerosal can be removed from these products without risking bacterial contamination, it makes sense to try to do so as part of a broader, precautionary effort to decrease mercury exposures in general -- not because thimerosal in vaccines has been proven to be dangerous. It has not.

To these ends, the committee makes the following recommendations about public health actions regarding thimerosal. The committee supports previous policies of government and medical advisory bodies that efforts be considered to decrease use of thimerosal in vaccines and drugs as much as possible. While the health effects of thimerosal are uncertain, the serious consequences of not getting vaccinated are very certain. This means that if thimerosal-free vaccines are not available, thimerosal-containing vaccines should be used. And finally, the committee encourages increased basic, clinical, and epidemiological research to better understand whether thimerosal may, in the past, have increased children's risk of developmental disorders.

Thimerosal-Containing Vaccines and Neurodevelopmental Outcomes Meeting Agenda

Cambridge, MA
Monday, July 16, 2001


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Due to the poor sound quality of some presentations, the meeting transcript has been made available in PDF file.

Welcome and Introductions

Marie McCormick, MD, ScD
Chair, Immunization Safety Review Committee

  • Risk Assessment of Thimerosal in Childhood Vaccines
    Leslie K. Ball, MD
    Division of Vaccines and Related Product Applications
    Office of Vaccines Research and Review
    Center for Biologics Evaluation and Research
    Food and Drug Administration<>
  • History of Thimerosal Concern and Comparative Policy Actions
    • Slides

  • Gary L. Freed, MD
    The Percy and Mary Murphy Professor of Pediatrics and Child Health Delivery
    Director, Division of General Pediatrics
    Professor of Health Management and Policy
    University of Michigan


  • Margie Andreae, MD
    Associate Director for Clinical Services
    Division of General Pediatrics
    University of Michigan


  • Biologic Plausibility and Planned Research
    Jane M. El-Dahr, MD
    Head, Section of Pediatric Allergy, Immunology, Rheumatology
    Tulane University Health Sciences Center<>



  • The Role of Heavy Metals in Autism
    Jeffrey Bradstreet, MD
    Director of Research
    The International Autism Research Center<>


  • Vaccine Safety Datalink (VSD) Screening Study and Follow-up Analysis with Harvard Pilgrim Data
    Thomas Verstraeten, MD, MSc
    EIS Officer, National Immunization Program
    Centers for Disease Control and Prevention  
    • Protocol for National Immunization Program Study on Thimerosal
      Paul Stehr-Green, DrPH, MPH
      Associate Professor of Epidemiology
      University of Washington School of Community Medicine and Public Health
      • William Thompson, PhD (for Q&A period only)
        Epidemiologist, National Immunization Program
        Centers for Disease Control and Prevention


      • Seychelles Islands Study
        Gary Myers, MD
        Professor of Neurology and Pediatrics
        University of Rochester School of Medicine and Dentistry<>


      • Faroe Islands Study
        Philippe Grandjean, MD, DMSc
        Institute of Community Health
        Odense University, Denmark
      • In Vitro Studies of Thimerosal Toxicity
        Boyd E. Haley, PhD
        Chairman and Professor, Department of Chemistry
        University of Kentucky<>  
      • Comparative Toxicology of Ethyl and Methyl Mercury
        George Lucier, PhD
        Former Director, Environmental Toxicology Program
        National Institute for Environmental Health Sciences<>  
      • Rising Incidence of Autism: Association with Thimerosal
        Mark F. Blaxill, MBA
        Autism Society of America, Massachusetts Chapter<>


      • NIH Studies on Thimerosal
        Polly Sager, PhD
        National Institute of Allergy and Infectious Diseases
        National Institutes of Health<>  
      • Wrap-up Commentary on Thimerosal Risk
        From the Perspective of a Developmental Toxicologist
        George Lambert, MD
        Environmental and Occupational Health Sciences Institute
        Robert Wood Johnson Medical School<>


      • Wrap-up Commentary on Thimerosal Risk from the Perspective of a Pediatric Vaccinologist
        Neal Halsey, MD
        Professor and Director
        Division of Disease Control
        Department of International Health
        Johns Hopkins University<>


      • Public comments

      Thimerosal-containing Vaccines and Neurodevelopmental Outcomes


      Agenda and Presentations

      On July 16, 2001 a scientific meeting examining thimerosal-containing vaccines and neurodevelopmental outcomes was held at the Charles Hotel in Cambridge, Massachusetts. The meeting agenda with speaker's slides, audio files of the presentations, and the meeting transcript is now available.
      The transcript is also available through the Public Access Records Office.

      Materials Reviewed by the Committee

      A Public Access File (PAF) has been created for the Immunization Safety Review Committee, which includes most written materials submitted to the committee from persons who are not officers, employees, or agents of the National Academies. The PAF may be reviewed by making an appointment with the Public Access Records Office (PARO) of the National Academies. The attached list includes most of the PAF records related to the July 16-18 meeting on Thimerosal-containing Vaccines and Neurodevelopmental Outcomes. Note that the PAF for this activity is updated continuously and may not be complete at the time of posting. Please check with the PARO if you have any questions regarding the PAF.

      Questions and Answers on the Toxicity of Ethylmercury

      One of the issues considered during the Immunization Safety Review Committee's meeting on thimerosal and neurodevelopmental outcomes is the toxicity of ethylmercury. As one of its data gathering activities, the committee asked Dr. László Magos to answer questions regarding the toxicity of ethylmercury. His answers and accompanying reference list are available here.

      The committee wishes to note that this represents the views of Dr. Magos and does NOT necessarily reflect the conclusions the committee will draw when it deliberates at its July meeting. The committee further notes that this paper is one source of many that it will review in the course of its deliberations.

      Reports and Summaries
      Thimerosal-containing Vaccines and Neurodevelopmental Disorders
      On October 1, 2001, the committee released its report on thimerosal-contaning vaccines. Below are links to the audio and written versions of the public release, full report text and Frequently Asked Questions.
      Press Release
      Opening Statement
      Listen to the Press Release
      Full-text of the report
      Executive Summary (PDF)
      Frequently Asked Questions about Thimerosal in Vaccines (PDF)

      Read it Online - FREE!

      Immunization Safety Review: Thimerosal - Containing Vaccines and Neurodevelopmental Disorders

      Kathleen Stratton, Alicia Gable, and Marie C. McCormick, Editors, Immunization Safety Review Committee, Board on Health Promotion and Disease Prevention


      Display Catalog Information
      Front Matter i-xiv
      Executive Summary 1-18
      Thimerosol-Containing Vaccines and Neurodevelopmental Disorders 19-19
      Origins of the Immunization Safety Review Project 20-21
      The Charge of the Committee 22-22
      The Study Process
      The Framework for Assessing Causality
      Under Review: Thimerosol Containing Vaccines and Neurodevelopmental Disorders 27-37
      Plausibility Assessment 38-64
      Causality Argument 65-65
      Significance Assessment 66-74
      Recommendations Regarding the Public Health Response 75-81
      Summary 82-85
      References 86-94
      Appendix A 95-106
      Appendix B 107-110
      Appendix C 111-116
      Appendix D 117-122
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      Information Available from the Public Access File

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      Materials reviewed by the committee for the Thimerosal and Neurodevelopmental Outcomes meeting
      Written materials presented to the Immunization Safety Review committee may be viewed by making an appointment with the Public Access Records Office (PARO) of the National Academies. Please e-mail or call PARO to make an inquiry, or to schedule an appointment to view materials. Normal business hours for PARO are 9:00 am to 5:00 p.m., Monday through Friday.

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