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Mercury
in Vaccines: What We Know
Because toxic
mercury exposure has a wide range of adverse health effects, currently in
the United States there is a public health effort to reduce human exposure
to mercury from all sources. (I) As
part of this effort, in July 1999, the U.S. Public Health Service (US PHS)
and the American Academy of Pediatrics (AAP) issued a joint statement
formally requesting that manufacturers eliminate or reduce the amount of thimerosal (a
mercury-containing compound) in vaccines. When the statement was issued,
available hepatitis B vaccines contained thimerosal, and concern arose that
newborn babies might be exposed to too much mercury if they got the
vaccine. For this reason, this joint statement also recommended that —
until a thimerosal-free hepatitis B vaccine became available — hepatitis B
immunization should be delayed until 2 to 6 months of age for infants born
to mothers who tested negative for the disease. Waiting until the newborns
were older gave them time to grow so that the amount of mercury would be
less in comparison to their body weight. (See the 1999 joint statement.)
On August
27, 1999, one thimerosal-free
formulation of hepatitis B vaccine was licensed, and on March 28, 2000 the
U.S. Food and Drug Administration approved a second preservative-free
pediatric hepatitis B vaccine. With the availability of these vaccines, in
July 2000, CDC recommended that infants routinely be immunized against
hepatitis B at birth. (See the CDC statement.)
However, the issue of thimerosal's safety as a preservative in some
vaccines has remained one of concern and confusion. While there is no
evidence that any child has been harmed by the mercury content of a
vaccine, some parents and health care providers still have questions: What
is thimerosal? Why is it in some vaccines? Does it present a risk to
children? Is it still in the vaccines that children receive?
What is
thimerosal, and why is it in some vaccines?
Thimerosal
is a compound that is 49.6% mercury by weight. Although it is not used in
all vaccines (for example, it is not used in measles-mumps-rubella or
chickenpox vaccines), it has been part of the manufacture of many vaccines
since the 1930s. Thimerosal has been used:
- to kill the bacteria
that make the vaccine itself (e.g., whole cell pertussis vaccine)
- to kill bacteria that
might enter the vaccine during the production process (e.g., influenza
vaccine)
- as a preservative to
prevent bacterial and fungal contamination of vaccines during their
clinical use. In this case, thimerosal is added at the end of the
production process either to the liquid vaccine itself or — in the
case of dry powder vaccines — to the liquid used to dilute the vaccine
Unless used
as a preservative, thimerosal contributes little to the final concentration
of thimerosal in vaccine (at most 2 to 3 micrograms of thimerosal per
milliliter of vaccine), so the chief concern has centered on thimerosal as
a preservative. (II) Although
preservatives are not required for single-dose vaccine vials, preservatives
are required to help prevent bacterial contamination of vaccine vials that
contain many doses (II) Why is this?
Most multi-dose vaccines come in vials that are topped with a rubber-like
stopper. With vials that contain many doses of vaccine, health care workers
repeatedly pass needles through the stopper when drawing up later vaccine
doses into the syringe and this can let bacteria enter the vial and
contaminate the vaccine.
Does thimerosal
in vaccines pose a risk to infants?
When
pregnant women eat foods or take medicines that contain mercury, the
mercury can be transferred to the developing fetus through the placenta.
Infants can be exposed to mercury through foods, including breast milk, or
medicines. Developing fetuses and young children are believed to be more
susceptible to mercury exposure than adults because mercury can interfere
with the developing nervous system. (I)
Guidelines
for safe exposure to methylmercury, based on the analysis of cases
where people were accidentally exposed to toxic levels of mercury, have
been developed by three federal agencies (II).
Although the three agencies' guidelines are each slightly different, each
leaves a large margin for safety, and exposure to amounts that exceed these
guidelines does not mean that the individual has been exposed to toxic
levels of mercury. Additionally, it should be noted that, some studies (III) show that ethyl mercury (the kind
to which thimerosal is metabolized) may be less toxic than methyl mercury
(the kind that was used in establishing the safety guidelines). (III) However, because no ethyl mercury
guidelines have been established, the analysis of thimerosal safety has
been based on methylmercury guidelines.
As part of
the Food and Drug Administration (FDA) Modernization Act of 1997, the FDA
began compiling a list of the amount and type of mercury in drugs and
foods. Notably, since the last formal FDA review of thimerosal use in
biologics in 1976, two important things have changed regarding vaccines:
there have been advances in the understanding of the human health effects
of low-level exposure to mercury, and there has been an increase in the
number of vaccines recommended for routine use in children (II). In their recent review, the FDA
found that, depending on which formulation an infant received for each of
his or her recommended vaccines, the infant could potentially be exposed to
total levels of mercury that would exceed the Environmental Protection
Agency (EPA) guideline of 0.1 micrograms of methylmercury per kilogram of
infant body weight per day. (See the National Academy of Science's National
Research Council July
2000 review of the EPA guideline.) This finding led to the request for
removal of thimerosal from vaccines and the temporary suspension of the
birth dose of hepatitis B vaccine until formulations of the vaccine became
available that did not contain thimerosal as a preservative.
Many
questions are being asked about the potential effect of thimerosal on the
developing fetus and infant, in particular on the developing nervous system
(IV). To begin, how is thimerosal
processed in the bodies of infants? In one recent study, scientists at the
University of Rochester Medical Center tested the blood levels of mercury
in 16 full-term infants shortly after the children had received recommended
vaccines that contained thimerosal. They found that "none of the blood
mercury levels observed in the studied infants exceeded the most recently
revised lowest level of maternal blood mercury considered to represent a
potentially significant exposure to the developing fetus." (V) More research is planned to evaluate
if the thimerosal in vaccines poses a risk to children.
Is
thimerosal still in the vaccines that children receive?
Currently,
all pediatric vaccines in the routine infant immunization schedule are
manufactured without thimerosal as a preservative. However, some pediatric
vaccines in distribution may still contain thimerosal as preservative
because the dating period for some vaccines is as long as 36 months. Other
vaccines (e.g., influenza vaccine; tetanus and diphtheria vaccine for older
children and adults) continue to be manufactured with thimerosal as a
preservative. For a current listing of the mercury concentration in most U.S.
licensed vaccines, you can access the website of the FDA or the Johns Hopkins University
Institute for Vaccine Safety.
The U.S.
Institute of Medicine (IOM) of the National Academy of Sciences – a
private, independent organization created by the federal government to be
an adviser on scientific and technological matters -- has established
an independent expert committee to review immunization safety concerns,
including thimerosal in vaccines. On October 1, 2001, the IOM
Immunization Safety Review Committee issued its report
“Thimerosal-Containing Vaccines and Neurodevelopmental Disorders,”
concluding, “The hypothesis that thimerosal exposure through the
recommended childhood immunization schedule has caused neurodevelopmental
disorders is not supported by clinical or experimental evidence.”
(Read the full report)In
addition, you can view the agenda, audiocast, slide presentations and a
transcript of a public
meeting the Committee held on July 16, 2001 to discuss available data
on thimerosal and nervous system disorders.
Other
websites of interest
Additional
References
American
Academy of Pediatrics, Committee on Infectious Diseases and Committee on
Environmental Health. (1999). Thimerosal in vaccines — An interim report to
clinicians. Pediatrics, 104(3), 570-574.
Brayden RM,
Pearson KA, Jones JS, Renfrew BL, and Berman S. (2001). Effect of
thimerosal recommendations on hospitals' neonatal hepatitis B vaccination
policies. Journal of Pediatrics, 138(5), 752-755.
Clark SJ,
Cabana MD, Malik T, Yusuf H, and Freed GL. (2001). Hepatitis B vaccination
practices in hospital newborn nurseries before and after changes in
vaccination recommendations. Archives of Pediatric and Adolescent Medicine,
155(8), 915-920.
Clements
CJ, Ball LK, Ball R, and Pratt RD. (2001). Thimerosal in vaccines: Is
removal warranted? Drug Safety, 24(8), 567-574.
Hurie MB,
Saari TN, and Davis JP. (2001). Impact of the joint statement by the American
Academy of Pediatrics/US Public Health Service on thimerosal in vaccines on
hospital infant hepatitis B vaccination practices. Pediatrics, 107(4),
755-758.
Mahaffey
KR. (1999). Methylmercury: A new look at the risks. Public Health Reports,
114(5), 396-399, 402-413.
Oram RJ,
Daum RS, Seal JB, and Lauderdale DS. (2001). Impact of recommendations to
suspend the birth dose of hepatitis B virus vaccine. Journal of the
American Medical Association, 285(14), 1874-1879.
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