Updated: August 6, 2008
This article answers the most frequently asked questions about thimerosal and its use as a preservative in vaccines.
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. 1
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:
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. 2
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 2
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.
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.1
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 agencies1. 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 3 5 show that ethylmercury (the kind to which thimerosal is metabolized) may be less toxic than methylmercury (the kind that was used in establishing the safety guidelines). However, because little information about ethylmercury has been available until recently, guidelines for thimerosal safety have 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 children2.
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 on an immunization day 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 also FDA’s response to a petiton to suspend and revoke all vaccines containing thimerosal for which there is a thimerosal-free replacement available).
This should have posed no risk to the child because the guidelines were established based on exposure to this amount of mercury every day. (See the National Academy of Science’s National Research Council July 2000 review of the EPA guideline.) Nevertheless, 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 6. To begin, how is thimerosal processed in the bodies of infants?
In one 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.” 7
More research is planned to evaluate if the thimerosal in vaccines poses a risk to children. The study also suggested that unlike the toxin methylmercury, thimerosal is eliminated through stools within two weeks.
Currently, all pediatric vaccines in the routine infant immunization schedule are manufactured without thimerosal as a preservative. As of January 14, 2003, the final lots of vaccines containing thimerosal as a preservative expired.
Other vaccines (for example, influenza vaccine; tetanus and diphtheria vaccine for older children and adults) continue to be manufactured with thimerosal as a preservative—although influenza vaccine without thimerosal preservative is also available.
Trivalent inactivated influenza vaccine (TIV) has recently been recommended for all children 6-23 months of age, in addition to the long-standing recommendations to give TIV to children and adults with certain medical conditions as well as older adults.
Thimerosal-free TIV is not available in the United States. However, TIV is available both with trace amounts of thimerosal and with thimerosal as a preservative.
The amount of thimerosal in current influenza vaccines is so low that it should not raise concern. The concern in 1999 regarding thimerosal in childhood vaccines was for their administration to infants in the first 6 months of life and reflected the possible cumulative total ethyl mercury burden from all the thimerosal-containing products administered at newborn, 2, 4 and 6 months of age.
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”.
Since that review, several new studies have looked for, but not found—an association of thimerosal exposure with autism and other developmental disorders (see References).
In 2004, the IOM Vaccine Safety Committee again began a review to consider any possible associations between vaccines and the occurrence of autism. The 2004 report, “Vaccines and Autism” states that “the body of epidemiological evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism.”
Guidelines for safe exposure to methylmercury are based on the analysis of unintended environmental exposures resulting in overt toxicity. Such guidelines have been developed by three federal agencies and the World Health Organization (WHO).
The World Health Organization recommends a limit of 3.3 micrograms of methylmercury/ kilogram of body weight/ week (0.47 micrograms/ kg/ day).
Three U.S. federal agencies have set lower guidelines for methylmercury exposure:
Although these guidelines are each slightly different, but each leaves a large margin for safety, and exposure to amounts that exceed these guidelines does not mean that the developing infant is exposed to toxic levels of mercury. 8
A concentration of 1:10,000
= 0.01% concentration
= 50 micrograms per 0.5 mL
= one millionth of a gram
Most pediatric vaccines come in doses of 0.5 mL (one-half milliliters) so most concentrations are reported “per 0.5 mL.”
Because thimerosal is half mercury, a vaccine with 0.01% concentration of thimerosal
= 0.005% concentration of mercury
= 25 micrograms of mercury per 0.5 mL of vaccine.
(For comparison, most commercial fish contain an average of 23 micrograms of mercury per 8 ounces of fish (i.e., 0.1 micrograms of mercury per gram of fish)).
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.
Parker SK, Schwartz B, Todd J, and Pickering LK (2004). Thimerosal-Containing Vaccines and Autistic Spectrum Disorder: A Critical Review of Published Original Data. Pediatrics, 114:793-804.