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Institute for Vaccine Safety

Johns Hopkins Bloomberg School of Public Health

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Thimerosal, a Mercury-containing Preservative used in Some Vaccines

July 8, 1999

The American Academy of Pediatrics and the US Public Health Service issued statements this week concerning thimerosal-containing vaccines and the combined levels of mercury in the currently recommended vaccines. The IVS believes that it is important to share all information with parents and health care providers so that they can make the most informed choices about how to minimize mercury exposure to children.

Neal Halsey, MD, the Director of the Institute for Vaccine Safety (IVS) at the Johns Hopkins School of Public Health, worked extensively with the American Academy of Pediatrics (AAP) to develop information about the safety of mercury-containing vaccines. The IVS supports the use of the safest possible vaccines for children.
The following supplemental information is provided by the Institute for Vaccine Safety to clarify some aspects of the new information about thimerosal and mercury in vaccines. More detailed information will be forthcoming within the next few weeks from a variety of sources. 

Not all vaccines contain thimerosal
Thimerosal is a mercury-containing preservative that has been used in some vaccines for many years. For each of the routinely recommended vaccines there are products available that do not contain mercury preservatives, including a hepatitis B vaccine (see table). Some vaccines in pre-filled syringes and all live vaccines like measles, mumps and rubella vaccine (MMR), varicella and other vaccines are preservative-free.
The joint statement from the AAP and US Public Health Service (PHS) indicates that very small infants who receive multiple doses of thimerosal-containing vaccines could receive more than the recommended limits for mercury exposure based on some of the recently developed guidelines. Therefore, the AAP and PHS have recommended delaying hepatitis B vaccination until 2-6 months of age (when the infant is larger). Also note that there is one hepatitis B vaccine that does not contain thimerosal; hopefully other thimerosal-free hepatitis B vaccines will be become available soon. The AAP will provide more information in the forthcoming guidelines.

Guidelines for mercury exposure
The Environmental Protection Agency, the FDA, The ATSDR, and the World Health Organization have developed guidelines for mercury exposure. These guidelines were written with the assumption that children will be exposed to low levels of mercury over a long period of time so that the concentration in the body builds up to steady-state level over time. The agencies responsible for the guidelines need to independently determine if the exposures from vaccines exceed this level and how these exposures should be counted.
At this time the IVS is not aware of any evidence that the amount of thimerosal in the vaccines has caused any harm except for mild allergic reactions. Nevertheless, we should reduce any potential risk however small.

Package Labels
The amount of mercury in some vaccines was underappreciated primarily because the package labeling listed the thimerosal concentration, but not the actual amount of mercury. A concentration of 1:10,000 seems like a very small amount, but this concentration provides 25 micrograms of mercury in a single dose of vaccine. Also, the term "mercury derivative" has been used to describe thimerosal, which may have been interpreted to mean that the product did not have the same biologic effects as mercury.

Environmental Exposure to Mercury
Since everyone is exposed to small amounts of mercury through foods and the environment, it is not possible to completely eliminate all exposures. The primary source of environmental mercury exposure is fish, especially predator fish like tuna, shark and red snapper. The Federal guidelines are based on studies of children born to women who had been exposed to high amounts of mercury during pregnancy in the Sechyelle and Faroe Islands and in Iraq. Scientists determined what level of maternal exposure during pregnancy was not associated with abnormalities detected by sophisticated neurologic and psychological testing in the children.
Mercury is transmitted across the placenta to the developing fetus and the fetal brain is much more sensitive to mercury exposure than the adult brain. Therefore, the guidelines are set to protect the developing fetus from mercury. The guidelines outline maximum daily exposures, but there is some uncertainty about how to interpret these guidelines with regard to cumulating or averaging exposures over time. The groups responsible for developing the guidelines will need to address this issue. 

Preservatives in Vaccines
Preservatives are needed in vaccines that come in multiple-dose vials to prevent the possibility of contamination by bacteria or fungi that could cause serious disease. Non-mercury containing preservatives have been developed and are used in several vaccines.
The IVS supports sharing all information with parents and health care providers so that they can make the most informed choices about how to minimize mercury exposure to children. The primary efforts at this time is to reduce mercury exposure for infants in the first six months of life, especially infants who are of low birth weight or premature.


This page was last updated on October 18, 2017

2016 Institute for Vaccine Safety