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IOM Scrutinizes Link Between Vaccines, Neurological Problems

July 19, 2001

By Mindy Hung

New York - Amidst concern about mercury content in vaccines commonly administered to children and a possible link to autism and attention deficit hyperactivity disorder (ADHD), the Institute of Medicine convened a meeting of their Immunization Safety Review Committee, in Cambridge, Massachusetts, on Monday.

The multidisciplinary committee, chaired by Marie McCormick, MD, ScD, of the Harvard School of Public Health, has been charged with assessing a possible causal relationship between inoculations and adverse health consequences, and disseminating the results of the assessment to the public.

Immunizations for Haemophilus influenzae type b (Hib), hepatitis B, diphtheria, and tetanus toxoids with acellular pertussis (DTaP) have been a source of controversy in recent years because they use thimerosal, a preservative containing low levels of ethylmercury (EtHg), which can cause neurological damage. Pregnant women and fetuses are at highest risk from exposure.

The metal is excreted from the body over time. However, according to a review conducted by the Food and Drug Administration in 2000, infants younger than 6 months may accumulate EtHg at levels that exceed federal safety guidelines established for the intake of another highly toxic mercury, methyl mercury, which occurs naturally in seafood.

Exposure to large amounts of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and a developing fetus. Irritability, shyness, tremors, changes in vision or hearing, and memory problems may result.

Although thimerosal has been used in biologics since the 1930s, an increased number of vaccinations combined with a sharp rise in reported autism and ADHD incidence has caused parents and some physicians to question the safety of the inoculations. Preliminary data from Centers for Disease Control and Prevention in Atlanta, Georgia, suggests a weak correlation between vaccine-related thimerosal exposure and incidence of language and speech delays, along with other neurological problems.

In a joint statement made in July 1999 and finalized in June 2000, the American Academy of Pediatrics, the US Public Health Service, and others emphasized that the risks of not vaccinating children far outweigh the possible problem of cumulative exposure to thimerosal.

They note, however that "because any potential risk is of concern, and the elimination of exposure to mercury in the form of thimerosal from vaccines is feasible, thimerosal should be removed from vaccines as soon as possible."

Except for DtaP, thimerosal-free versions of pediatric vaccines are available.

The measles-mumps-rubella vaccine, which has been implicated in the rise in cases of autism, does not contain thimerosal.

The Immunization Safety Review Committee will release its findings on its Web site.

Mindy Hung is an associate editor at Medscape.

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