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News from the National Academies

Date: Oct. 1, 2001
Contacts: Saira Moini, Media Relations Officer
Cory Arberg, Media Relations Assistant
(202) 334-2138; e-mail <news@nas.edu>

For Immediate Release

Link Between Neurodevelopmental Disorders and Thimerosal Remains Unclear
WASHINGTON -- Current scientific evidence neither proves nor disproves a link between the mercury-containing preservative thimerosal and neurodevelopmental disorders in children, says a new report from the Institute of Medicine of the National Academies. While very few vaccines given to children in the United States today still contain thimerosal, prudence dictates that precautionary measures be taken to decrease thimerosal exposure even further.

Thimerosal is used in some vaccines and other pharmaceutical products to prevent bacterial contamination. Vaccines against measles, mumps, and rubella; varicella; and polio have never contained the preservative. However, until recently, several other vaccines on the recommended childhood immunization schedule in the United States did. They are now manufactured without thimerosal, but an unknown, probably small number of vaccine doses for hepatitis B; diphtheria, tetanus, and pertussis; and haemophilus influenzae type B, a form of bacterial meningitis, are still on clinic shelves. These supplies should not be used when alternatives are available, said the committee that wrote the report.

"Most children in the United States being immunized today and in the future are unlikely to receive a vaccine that contains thimerosal," said committee chair Marie McCormick, professor of maternal and child health at Harvard School of Public Health, Boston. "In those few cases where only supplies containing the preservative are available, the vaccines should be administered rather than foregoing immunization. While the health effects of thimerosal are uncertain, we know for sure that these vaccines protect against real, proven threats to unvaccinated infants, children, and pregnant women."

A connection between exposure to certain forms of mercury and nervous system abnormalities has long been recognized. People exposed to high mercury levels can experience difficulties with coordination, vision, and learning. Most studies of the effects of low-level exposure have focused on methylmercury from fish and seafood products. Thimerosal contains a different chemical form called ethylmercury.

The committee's comprehensive assessment of the scientific literature on thimerosal included analyses of published and unpublished studies proposing an association with disorders such as autism, and it found them to be inconclusive. No evidence currently exists that proves a link between thimerosal-containing vaccines and autism, attention deficit-hyperactivity disorder, speech or language delays, or other neurodevelopmental disorders.

Mercury can build up in the body with each additional exposure, whether from vaccinations or other sources, such as fish consumption. It is medically plausible that some children's risk of a neurodevelopmental disorder could rise in part through increased mercury exposure from thimerosal-containing vaccines. Because safety guidelines were established specifically for methylmercury, however, it is not clear whether additional exposure from ethylmercury could result in an unsafe cumulative level.

However, as another precaution, policy-makers in the United States should consider changing existing policies to reduce exposure to thimerosal as much as possible. For example, professional societies and government agencies should review their policies about nasal sprays, eye drops, and other products that contain thimerosal and are used for infants, children, and pregnant women, the report says.

For more than half a century, thimerosal was added to some vaccines that protected children against serious diseases. In 1999 the U.S. Public Health Service, the American Academy of Pediatrics, and the American Academy of Family Physicians issued precautionary recommendations limiting mercury exposure of infants and young children, a measure that prompted development of thimerosal-free versions of routine childhood vaccines. By mid-2000, thimerosal-free vaccines against hepatitis B and bacterial meningitis were widely available. A combination vaccine for diphtheria, pertussis, and tetanus also is available today without thimerosal.

The preservative is still used in a few vaccines, including influenza vaccine, which is given annually during the viral flu season to adults and some children. The Centers for Disease Control and Prevention recommend that protecting pregnant women and high-risk children during flu season take precedence over any possible risk from thimerosal exposure.

Public trust in vaccine safety must be maintained, the committee said. To this end, it is important to understand more fully the possible effects of thimerosal. Future research should include population studies of the occurrence of neurodevelopmental disorders before and after thimerosal was removed from most vaccines. Levels of women's prenatal and postnatal mercury exposure from medicinal products and sources such as fish consumption should be examined as well. Clinical research also should examine how the bodies of children, including those diagnosed with neurodevelopmental disorders, absorb and process heavy metals like mercury and which medical therapies are effective in ridding the body of them. This second study in a series on vaccine safety was sponsored by the Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases. The Institute of Medicine is a private, nonprofit institution that provides health policy advice under a congressional charter granted to the National Academy of Sciences. A committee roster follows.

Copies of Thimerosal-Containing Vaccines and Neurodevelopmental Disorders are available from the National Academy Press; tel. (202) 334-3313 or 1-800-624-6242 or on the Internet at http://www.nap.edu. The cost of the report is $25.00 (prepaid) plus shipping charges of $4.50 for the first copy and $.95 for each additional copy. Reporters may obtain a copy from the Office of News and Public Information (contacts listed above).


INSTITUTE OF MEDICINE
Board on Health Promotion and Disease Prevention

Immunization Safety Review Committee


Marie McCormick, M.D., Sc.D.* (chair)
Professor and Chair
Department of Maternal and Child Health
Harvard School of Public Health
Boston

Ronald Bayer, Ph.D.
Professor
Division of Sociomedical Sciences
Joseph L. Mailman School of Public Health
Columbia University
New York City

Alfred Berg, M.D., M.P.H.*
Professor and Chair
Department of Family Medicine
University of Washington School of Medicine
Seattle

Rosemary Casey, M.D.
Associate Professor of Pediatrics
Jefferson Medical College, and
Pediatrician
Lankenau Faculty Pediatrics
Wynnewood, Pa.

Joshua Cohen, Ph.D.
Senior Research Associate
Harvard Center for Risk Analysis
Harvard School of Public Health
Boston

Vernice Davis-Anthony, M.P.H., R.N.
Senior Vice President
Corporate Affairs and Community Health
St. John Health System
Detroit

Betsy Foxman, Ph.D.
Professor
Department of Epidemiology
University of Michigan School of Public Health, and
Director
Center for Molecular and Clinical Epidemiology of Infectious Diseases
Ann Arbor, Mich.

Constantine Gatsonis, Ph.D.
Professor of Medical Science and Applied Mathematics, and
Director, Center for Statistical Sciences
Brown University
Providence, R.I.

Steven Goodman, M.D., M.H.S., Ph.D.
Associate Professor of Oncology, Pediatrics, Epidemiology, and Biostatistics
Johns Hopkins Schools of Medicine and Public Health
Baltimore

Ellen Horak, R.N., M.S.N.
Chief of Local Health Services
Office of Local and Rural Health
Kansas Department of Health and Environment
Topeka

Michael Kaback, M.D.*
Professor of Pediatrics and Reproductive Medicine
University of California
San Diego

Gerald Medoff, M.D.
Professor of Medicine and Microbiology and Immunology, and Senior Adviser to the Chairman of the Internal Medicine Department
Washington University School of Medicine
St. Louis

Rebecca Parkin, Ph.D., M.P.H.
Associate Research Professor
Department of Occupational and Environmental Health
School of Public Health and Health Services
George Washington University Medical Center
Washington, D.C.

Bennett A. Shaywitz, M.D.
Professor of Pediatrics and Neurology and Chief of Pediatric Neurology
Yale University School of Medicine, and
Co-Director
Yale Center for the Study of Learning and Attention
New Haven, Conn.

Christopher Wilson, M.D.
Professor and Chair
Department of Immunology
University of Washington
Seattle

INSTITUTE STAFF

Kathleen Stratton, Ph.D.
Study Director


* Member, Institute of Medicine