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The committee's comprehensive assessment of the scientific literature on thimerosal included analyses of published and unpublished studies proposing an association with disorders such as autism, and it found them to be inconclusive.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.

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