Recent Congressional hearings have re-focused attention on the possible health consequences of mercury exposure. Therefore, NNii has updated its information on the use of thimerosal, a derivative of mercury, in vaccines.
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. (1) 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. (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.
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. (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 agencies (2). 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) 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). (3) 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 (2). 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 (4). 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." (5) 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
American Academy of Pediatrics statement on mercury in the environment
American Academy of Pediatrics statement on the October 2001 IOM report
The CDC's National Immunization Program information on thimerosal in vaccines
Food and Drug Administration information on trace amounts of thimerosal in new DTaP vaccine
Food and Drug Administration general information on thimerosal
Immunization Action Coalition general information and resources on thimerosal
- Johns Hopkins University Institute for Vaccine Safety references and useful links
- National Institue of Allergy and Infectious Diseases research on thimerosal
National Partnership for Immunization thimerosal section
- World Health Organization questions and answers on thimerosal
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.