Thimerosal
Vaccine manufacturers who produce multidose vaccine
vials use thimerosal as a preservative. Thimerosal is approximately
50% mercury by weight, and it has been one of the
most widely used preservatives in vaccines. It is metabolized
or degraded to ethylmercury and thiosalicylate. Ethylmercury
is an organomercurial that should be distinguished
from methylmercury, a related substance that has been the
focus of considerable study (Thimerosal in Vaccines, n.d.).
Methylmercury is bioavailable and can accumulate in the
brain and cause neurologic damage. The ethylmercury found
in thimerosal is not bioavailable. In studies, ethylmercury
does not accumulate in the body or the brain and is metabolized
and cleared by the body. (Burbacher, Shen, Liberato,
Grant, & Cernichiari, 2005).
Thimerosal has antimicrobial qualities that keep vaccines
safe from inadvertent contamination through routine
multiple punctures in a vial. Thimerosal had been used by
vaccine manufacturers for years but came under scrutiny in
1999, as discussed earlier in this article. At that time, the
FDA and the CDC published statements that indicated
manufacturers should reduce or eliminate the amount of
thimerosal used in vaccines. The CDC further recommended
the birth dose of hepatitis B vaccine be suspended
for infants until thimerosal-free vaccine was available (CDC,
1999b).
The CDC stated:
. . . given the widely acknowledged value of reducing
exposure to mercury, vaccine manufacturers, the FDA,
and other Public Health Service (PHS) agencies are collaborating
to reduce the thimerosal content of vaccines or
to replace them with formulations that do not contain
thimerosal as a preservative as soon as possible without
causing unnecessary disruptions in the vaccination
system. The FDA will expedite review of supplements to
manufacturers’ product license applications that present
formulations for eliminating or reducing the mercury
content of vaccines. (CDC, 1999, p. 997)Vaccine manufacturers then worked to assure removal of
thimerosal from vaccines. By 2001, all vaccines routinely recommended
for children 6 years of age and under in the
United States were produced without thimerosal as a preservative,
with the exception of some doses of inactivated influenza
vaccine. Today, all vaccines are available without
thimerosal, including several influenza vaccine presentations
(e.g., single-dose prefilled syringes and the intranasal
vaccine).
Many studies have been undertaken to examine the risks
associated with thimerosal in vaccines. In 2003, Stehr-Green
et al. assessed autism incidence and the use of thimerosalcontaining
vaccines:
“Data did not support an association
between thimerosal-containing vaccines and autism in
Denmark and Sweden where exposure to thimerosal was
eliminated in 1992 and where autism rates continued to
increase” (Stehr-Green et al., 2003, p. 106).Another study in 2003 utilized the Vaccine Safety Datalink
(VSD) to screen for possible associations between exposure
to thimerosal-containing vaccines and a variety of renal, neurologic,
and developmental problems: “No consistent significant
associations were found between thimerosal-containing
vaccines and neurodevelopmental outcomes” (Verstraeten
et al., 2003, p. 1,042).
The CDC conducted a follow-up study to the Verstraeten
et al. VSD study. This was a large study that also utilized the
VSD data to investigate a possible link between thimerosal in
vaccines and childhood developmental concerns. An excerpt
from the study finding reads:
. . . some people believe increased exposure to thimerosal
(from the addition of important new vaccines recommended
for children) explains the higher prevalence in
recent years. However, evidence from several studies
examining trends in vaccine use and changes in autism
frequency does not support such an association. Furthermore,
a scientific review by the Institute of Medicine
(IOM) concluded that “the evidence favors rejection of a
causal relationship between thimerosal-containing vaccines
and autism.” (CDC, 2007, p. 144.)Thompson et al. (2007) further examined the hypotheses
that “increasing exposure to thimerosal is associated with
neurodevelopmental disorders. Findings did not support a
causal association between early exposure to mercury from
thimerosal-containing vaccines and immune globulins and
deficits in neuropsychological functioning at the age of 7 to
10 years” (Thompson et al., p. 1,290).
PMID: 19614825 [PubMed - indexed for MEDLINE]
http://www3.interscience.wiley.com/journal/122504388/abstract
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