Vaccine Safety and Thimerosal
Thimerosal is a preservative that prevents bacteria from growing. Thimerosal has
been widely used since the 1930s (over 75 years) in vaccines and other
medications, to help prevent contamination with harmful bacteria and fungi.
The Food & Drug Administration (FDA) considers the small amount of
thimerosal in some vaccines to be acceptably safe.
Thimerosal contains mercury. If you were to weigh a thimerosal molecule, about
half its weight would be mercury. The mercury in thimerosal is in a form called
ethylmercury. This is different from another form of mercury called
methylmercury.
High levels of methylmercury in the body can harm the kidneys or nerve tissue.
Humans are exposed to methylmercury when they eat certain kinds of fish or
seafood.
On the other hand, the human body removes (“eliminates”) ethylmercury more
rapidly than methylmercury. Because of this, ethylmercury does not build up in
the body, reducing its ability to cause harm. If there were any theoretical
danger from thimerosal, it would apply to young infants and pregnant women, but
not to people with larger body weights (such as older children and adults).
Even though there is little evidence that thimerosal can be harmful, US vaccine
manufacturers have either reduced or removed thimerosal from childhood
vaccines. This decision was made to make vaccines as safe as possible.
Very few vaccines continue to contain thimerosal as a preservative. A list of
thimerosal content in some US vaccines is available from:
http://www.vaccinesafety.edu/thi-table.htm
In developing countries, thimerosal-containing vaccines remain in wide use
because thimerosal-free vaccines are often more expensive.
Multiple population-based studies show no association between immunization with
thimerosal-containing vaccines and specific neurodevelopmental disorders (for
example, autism, speech or language delay, attention deficit disorder).
Immunization with vaccines containing thimerosal continues to offer the full
value of the vaccine without any measurable risk from mercury. The benefits of
immunization far outweigh any potential risks from exposure to
thimerosal-containing vaccines, in the considered opinion of both the FDA and
the Centers for Disease Control & Prevention (CDC).
The Institute of Medicine's Immunization Safety Review Committee (an independent
committee of expert physicians and scientists) recently released a report,
Vaccines and Autism, which concluded, "the body of epidemiologic evidence
favors rejection of a causal relationship between thimerosal-containing
vaccines and autism." Available from:
http://www.nap.edu/catalog/10997.html
Four large studies have compared the risk of autism in children receiving
vaccines containing thimerosal to those who received vaccines without
thimerosal. Each study found no association between thimerosal exposure and
autism or other neurodevelopmental disorders (NDD). These studies are clear,
consistent, and reproducible and show that exposure to thimerosal through
childhood vaccines does not cause autism or NDD. The only evidence of harm
caused by thimerosal is a small risk of allergy, skin rash, or swelling at the
injection site, similar to other vaccines.
Additional information on vaccines and thimerosal is available from Immunization
Action Coalition: http://www.immunize.org/catg.d/p2066.pdf
Centers for Disease Control and Prevention: http://www.cdc.gov/nip/vacsafe/concerns/thimerosal/faqs-thimerosal.htm#1
National Network for Immunization Information:
http://www.immunizationinfo.org/thimerosal_mercury_issues.cfm
Letter to Congress from Multiple National Organizations that Support Safe and
Effective Vaccines:
http://www.immunize.org/thimerosalletter.pdf
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