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History in vaccines
Recent changes
Institute of Medicine reports
References
History in vaccines
Thimerosal is a mercury-containing compound that kills or prevents the
growth of microorganisms. Since the 1930’s, thimerosal has been used as a
preservative to prevent growth of bacteria and fungi in vaccines (FDA 2005).
One of the breakdown products of thimerosal is ethylmercury, an organic form
of mercury. The Food and Drug Administration (FDA) Modernization Act of 1997
called for the FDA to review and assess the risk of all mercury-containing
food and drugs, including vaccines (CDC 1999). The FDA noted that due to
immunization, a 6-month old infant could potentially be exposed to
cumulative levels of ethylmercury exceeding Environmental Protection Agency
(EPA) guidelines for safe intake of methylmercury, a related mercury
compound that has been more thoroughly studied than ethylmercury (CDC 2003).
It is not known if toxicity for ethylmercury is the same as that for
methylmercury, but in conducting safety evaluations, the FDA assumes that
the two chemicals have equal toxicity (FDA 2005). According to a joint
statement issued in 1999 by the American Academy of Pediatrics (AAP), the
United States Public Health Service (PHS), and the American Academy of
Family Physicians, there was no evidence of harm resulting from exposure to
the low levels of thimerosal used in some vaccines (CDC 1999). However, the
statement called for the removal of thimerosal from vaccines routinely
recommended for infants as a precautionary measure.
Recent changes
With the exception of inactivated influenza vaccines, none of the
vaccines routinely recommended for children 6 years and younger currently
contain thimerosal as a preservative (FDA 2005). Some of those vaccines
still contain trace amounts of thimerosal that is added during their
manufacture. However, mercury exposures resulting from use of thimerosal in
vaccine manufacturing processes are 50 times lower than exposures resulting
from use of thimerosal as a preservative. Based on the removal or reduction
of thimerosal in vaccines, it has been estimated that the maximum
ethylmercury exposure an infant could receive through vaccines has been
reduced by 95%. Limited supplies of thimerosal-free or thimerosal-reduced
influenza vaccines are available for use in children and pregnant women and
the FDA is working with manufacturers to increase those supplies in the
future.
Institute of Medicine reports
Exposure to high levels of mercury can clearly damage the nervous system
but harmful effects of exposure to low levels of mercury, such as those
associated with use of thimerosal as a preservative in vaccines, have not
been demonstrated (IOM 2001). The National Academy of Sciences Institute of
Medicine (IOM) assembled a committee of scientists to examine the health
implications of thimerosal in vaccines (IOM 2004a). The first IOM report was
released in 2001 (IOM 2001). There were very few studies for the committee
to review. The most thorough study consisted of data collected by the
Centers for Disease Control (CDC) and seven health maintenance organizations
(HMOs). The committee concluded that there was insufficient evidence to
determine if childhood thimerosal exposure causes neurological disorders
such as autism, attention deficit hyperactivity disorder (ADHD), and speech
or language delay. The committee stated that the theory that thimerosal
could be associated with neurological disorders in children was biologically
possible and noted that more studies were needed. The Committee further
stated that the decision to remove thimerosal from vaccines was a prudent
action in supporting public health goals of reducing mercury exposures in
infants and children.
The second report of the IOM Committee was issued in 2004 (IOM 2004b).
Autism was the focus of this report. The committee reviewed several studies
that were conducted following the first review, including studies from the
U.S., Denmark, Sweden, and the United Kingdom. The committee concluded that
the body of evidence suggested that thimerosal-containing vaccines did not
cause autism. Studies designed to determine possible biologic events in the
development of autism following vaccination were also reviewed. The
committee concluded there is no evidence that thimerosal in vaccines affects
biological processes possibly related to the development of autism.
According to the IOM (IOM 2004b), there is no need for a review of
policies for recommendations or schedules of routine childhood vaccines.
References
CDC 1999. Notice to Readers: Thimerosal in Vaccines: A Joint Statement of
the American Academy of Pediatrics and the Public Health Service. MMWR
48(26):563-565 -
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4826a3.htm.
CDC 2003. CDC research on the safety of thimerosal containing vaccines -
http://www.cdc.gov/nip/vacsafe/concerns/thimerosal/researchQAs.htm.
FDA 2005. Thimerosal in vaccines -
http://www.fda.gov/cber/vaccine/thimerosal.htm#1.
IOM 2001. Immunization Safety Review: Thimerosal-containing vaccines and
neurodevelopmental disorders -
http://books.nap.edu/execsumm_pdf/10208.pdf.
IOM 2004a. Immunization safety review -
http://www.iom.edu/project.asp?id=4705.
IOM 2004b. Immunization Safety Review: Vaccines and Autism (Free
Executive Summary) -
http://www.nap.edu/execsumm_pdf/10997.pdf.
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