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Understanding Thimerosal,

For more information on vaccines,

vaccine-preventable diseases, and

Mercury, and Vaccine Safety

vaccine safety:

Last reviewed Februar y 2013

Thimerosal is a mercur y-containing compound that

prevents the growth of dangerous bacteria and | questions and answers |
fungus. It is used as a preservative for flu vaccines What is thimerosal? Is it the same as mercury?
in multi-dose vials, to keep the vaccine free from Thimerosal is a compound that contains mercury. Mercury is a metal
contamination. Thimerosal is also used during the found naturally in the environment.
manufacturing process for some vaccines to prevent
the growth of microbes. Why is thimerosal used in some vaccines?
Because it prevents the growth of dangerous microbes, thimerosal
In 1999, as a precautionar y measure, the U.S. is used as a preservative in multi-dose vials of flu vaccines, and
Public Health Ser vice recommended removing in two other childhood vaccines, it is used in the manufacturing
thimerosal as a preservative from vaccines to process. When each new needle is inserted into the multi-dose
reduce mercury exposure among infants as much vial, it is possible for microbes to get into the vial. The preservative,
as possible. thimerosal, prevents contamination in the multi-dose vial
Today, except for some flu vaccines in multi-dose when individual doses are drawn from it. Receiving a vaccine
vials, no recommended childhood vaccines contain contaminated with bacteria can be deadly.
thimerosal as a preservative. For two childhood vaccines, thimerosal is used to prevent the growth
In all other recommended childhood vaccines, no of microbes during the manufacturing process. When thimerosal
thimerosal is present, or the amount of thimerosal is used this way, it is removed later in the process. Only trace (very
tiny) amounts remain. The only childhood vaccines today that
is close to zero.
have trace amounts of thimerosal are one DTaP and one DTaP-Hib
No reputable scientific studies have found an combination vaccine.
association between thimerosal in vaccines
and autism. Why was thimerosal removed from vaccines given

There are two different compounds that contain to children?

In 1999, the Food and Drug Administration (FDA) was required by

mercury: ethylmercur y and methylmercury. The low
law to assess the amount of mercury in all the products the agency
levels of ethylmercury in vaccines are broken down
oversees, not just vaccines. The U.S. Public Health Service decided
by the body differently and clear out of the blood that as much mercury as possible should be removed from vaccines,
more quickly than methylmercury. and thimerosal was the only source of mercury in vaccines. Even

Food and Drug Public Health Service Manufacturers Manufacturing All childhood vaccines Influenza vaccination Influenza vaccination
Administration (FDA) (PHS) and American begin to phase of childhood with thimerosal as recommended for U.S. recommended for
announces results of its Academy of Pediatrics out use of vaccines with a preservative have children ages 6 through all U.S. children ages
review of total amount (AAP) recommend thimerosal as thimerosal as passed their expiration 23 months old. Thimerosal 6 months through
of ethylmercury from removing thimerosal a preservative a preservative date and are no longer is used as a preservative in 18 years.
thimerosal preservative as a preservative from in childhood ceases. available in the U.S. The multi-dose vials to prevent
in childhood vaccines. childhood vaccines as a vaccines. amount of mercury in contamination of the vial
precautionary measure. vaccines recommended when individual vaccine
for children is close doses are drawn from it.
to zero. Single-dose vials without
thimerosal are available.













| questions and answers | continued
though there was no evidence that thimerosal in vaccines was past, the vaccines were put into multi-dose vials, which could
dangerous, the decision to remove it was a made as a precautionary become contaminated when new needles were used to get vaccine
measure to decrease overall exposure to mercury among young infants. out of the vial for each dose.
This decision was possible because childhood vaccines could be
reformulated to leave out thimerosal without threatening their safety, Was thimerosal used in all childhood vaccines?
effectiveness, and purity. No. A few vaccines contained other preservatives, and they still
do. Some other vaccines, including the measles, mumps, and
Today, no childhood vaccine used in the U.S.except some rubella vaccine (MMR) never contained any preservative or
formulations of flu vaccine in multi-dose vialsuse thimerosal
any mercury.
as a preservative.

Why is thimerosal still in some flu vaccines that children | the science |
may receive?
The studies below are examples of some of the different
To produce enough flu vaccine for the entire country, some of it methods that researchers have used to examine thimerosal
must be put into multi-dose vials. When each individual vaccine dose safety in vaccines. Researchers have looked at very large
is drawn from the vial with a fresh needle, it is possible for microbes to groups, such as all children born in a six-year period in
get into the vial. So, this preservative is needed to prevent contamination Denmark, as well as smaller, defined groups, such as children
of the vial when individual doses are drawn from it. Children can safely diagnosed with autism in California. In some of the studies,
researchers compared rates of autism among those who were
receive flu vaccine that contains thimerosal. Flu vaccine in single-dose
vaccinated with thimerosal-containing vaccines and those who
vials that does not contain thimerosal also is available. were not. Researchers consistently found that children who
received thimerosal in vaccines were not more likely to have
Was thimerosal in vaccines a cause of autism? autism than those who did not.
Reputable scientific studies have shown that mercury in vaccines given
Thimerosal Exposure in Infants and Developmental Disorders:
to young children is not a cause of autism. A Retrospective Cohort Study in the United Kingdom Does
The studies used different methods. Some examined rates of autism in Not Support a Causal Association by Nick Andrews et al.
Pediatrics. September 2004. Vol 114: pages 584591.
a state or a country, comparing autism rates before and after thimerosal
was removed as a preservative from vaccines. In the United States and full/114/3/584
other countries, the number of children diagnosed with autism has not
Pervasive Developmental Disorders in Montreal, Quebec,
gone down since thimerosal was removed from vaccines. Canada: Prevalence and Links with Immunizations by Eric
Frombonne et al. Pediatriacs. July 2006. Vol 118: e139-e150.
What keeps todays childhood vaccines from becoming
contaminated if they do not contain thimerosal as a 1/e139
preservative? Association between Thimerosal-Containing Vaccine and
The childhood vaccines that used to contain thimerosal as a preservative Autism by Anders Hviid et al. Journal of the American Medical
are now put into single-dose vials, so no preservative is needed. In the Association. October 2003. Vol 290: pages 17631766.
Immunization Safety Review: Vaccines and Autism.
What is the difference between ethylmercury
Institute of Medicine. The National Academies Press: 2004.
and methylmercury?
When learning about thimerosal and mercury it is important to Review-Vaccines-and-Autism.aspx
understand the difference between two different compounds that Prenatal and Infant Exposure to Thimerosal from Vaccines
contain mercury: ethylmercury and methylmercury. They are totally and Immunoglobulins and Risk of Autism by Cristofer Price
different materials. Methylmercury is formed in the environment et al. Pediatrics. September 2010. Vol 126: pages 656-664.
when mercury metal is present. If this material is found in the body,
it is usually the result of eating some types of fish or other food. 0309v1
High amounts of methylmercury can harm the nervous system. This Continuing Increases in Autism Reported to Californias
has been found in studies of some populations that have long-term Developmental Services System by Robert Schechter et
exposure to methylmercury in foods at levels that are far higher than al. Archives of General Psychiatr y. January 2008. Vol 65:
pages 19-24.
the U.S. population. In the United States, federal guidelines keep as
much methylmercury as possible out of the environment and food,
but over a lifetime, everyone is exposed to some methylmercury. Early Thimerosal Exposure and Neuropsychological Outcomes
at 7 to 10 Years by William Thompson et al. The New England
Ethylmercury is formed when the body breaks down thimerosal.
Journal of Medicine. September 2007. Vol 357: pages 1281
Low-level ethylmercury exposures from vaccines are very different 1292.
from long-term methylmercury exposures because ethylmercury is
broken down by the body differently and clears out of the blood
more quickly. For more information on vaccines call 800-CDC-INFO
(800-232-4636) or visit