Thiomersal Fact Sheet

Thiomersal (also known as thimerosal) is a mercury-based preservative used in some vaccines. The level of mercury in vaccines is very low and there is no evidence that thiomersal in vaccines has caused any health problems except minor reactions, such as redness at the injection site. However, because of the potential risk of harm from mercury, thiomersal was removed from most childhood vaccines as a precautionary measure. Follow up studies in children and adults have not shown any harmful effects from thiomersal in vaccines.

What is thiomersal?

Thiomersal, also known as thimerosal, is an organic compound containing 49.6% ethylmercury by weight. It has been used in very small amounts in some vaccines since the 1930s to prevent bacterial and fungal contamination, particularly in multidose vials where withdrawing repeated doses from the same vial was more likely to result in contamination.

In 1999 concerns were raised in the United States that the total amount of mercury, from thiomersal in vaccines given in the infant immunisation schedule, would potentially exceed the recommended level set by a US government agency. There were no studies indicating that the ethylmercury in thiomersal had caused harmful effects in children (except for occasional redness at the injection site), however it was recommended that thiomersal be removed from many childhood vaccines to eliminate any potential risk. Since that time much more information has been gathered regarding thiomersal (ethylmercury).

What is mercury?

Mercury is a metal occurring naturally in the environment. Mercury is found in three main forms: metallic mercury which gives rise to mercury vapour, inorganic mercury (a form in the environment and in animal tissues) and organic mercury (the two main forms of which are methylmercury and ethylmercury). These various forms of mercury are found in the air, earth, aquatic sediment, in fish (particularly in long lived fish such as sharks), and are used in industrial processes, dental fillings, thermometers, and vaccines.

The two organic forms of mercury, methylmercury and ethylmercury (in thiomersal), are closely related but they have important differences. Methylmercury is more potent; it accumulates in the body because the time taken for the body to eliminate it (know as the half life) is about 50 days. Ethylmercury (in thiomersal) does not accumulate in the body to such an extent, because its half life is only about 7-10 days. Ethylmercury is rapidly converted in the body to inorganic mercury, which is excreted in the stool. Mercury can have harmful effects on the central nervous system, skin and kidneys, but most cases of the toxic effects of mercury have been from methylmercury, not ethylmercury.

How much mercury is harmful?

Mercury is harmful only after it reaches a certain level in the body. The toxicity depends on the amount of mercury consumed in relation to body weight, over a period of time. Therefore, because of their size, infants are at greater risk than adults. Different expert bodies have determined that safe levels of mercury consumption lie somewhere between 0.7 µg/kg body weight/week (Environmental Protection Agency, USA) to 3.3 µg/kg of body weight/week (World Health Organization). These values indicate levels of exposure that can be tolerated, and have been deliberately calculated to be much lower than the level at which harm might occur. For example the EPA level is ten times below the lowest level calculated as causing harm, so there is a large built-in safety margin. In addition, these levels refer to methylmercury, whereas thiomersal is converted to ethylmercury, which is broken down and excreted more rapidly and does not accumulate in the body like methylmercury.

How much mercury exposure results from vaccines?

In Australia, thiomersal has been removed from all routine childhood vaccines since 2000.The exception is one type of Hepatitis B vaccine which contains a greatly reduced amount of thiomersal (see Table 1 below). When thiomersal-containing vaccines were being used before 2000, the maximum number of doses of thiomersal-containing vaccines a 6 month old child might have received was as follows: 3 doses each of diphtheria-tetanus-pertussis vaccine, 3 doses of hepatitis B vaccine, and 3 doses of Hib vaccine. This would have resulted in a total intake of 175 µg of ethylmercury, which is equivalent to about 1.9 µg/kg body weight per week, for an average-sized baby. This level is well below the World Health Organization (WHO) limit for methylmercury discussed above. Two studies measuring mercury levels in the blood in infants given thiomersal-containing vaccines have indicated that their blood concentrations of mercury did not rise above designated levels, except possibly transiently in a premature infant less than 1kg in weight.

In many countries thiomersal continues to be used in vaccines. The Global Advisory Committee on Vaccine Safety (GAVSC) of the WHO has concluded that "there is currently no evidence of mercury toxicity in infants, children or adults exposed to thiomersal-containing vaccines" and that "there is no reason to change current immunisation practices with thiomersal-containing vaccines on the grounds of safety".

What studies have been done to look at the health effects of thiomersal in vaccines?

Many studies in Denmark, Sweden, the United States, and the United Kingdom have now shown that there is no evidence of developmental or neurologic abnormalities resulting from the use of vaccines containing thiomersal. In 2004 a report by the Institutes of Medicine, an independent expert body in the United States, concluded that there is no association between autism and vaccines that contain thiomersal. Also in 2004 an extensive review of all the studies on thiomersal-containing vaccines and autism and neurodevelopmental disorders was published in the journal Pediatrics. Studies looking at autism, mental retardation, speech disorders, and attention deficit disorder, as well as other conditions were reviewed. Overall, the evidence indicated that autism and neurodevelopmental disorders are not associated with thiomersal in vaccines. The reviewers noted that the epidemiologic studies done that suggest a link (notably only by one pair of authors) “have significant design flaws that invalidate their conclusions.”

Why was thiomersal removed from childhood vaccines if there is no danger?

Although there has been a lack of evidence that thiomersal in vaccines is harmful, the recommendations to remove it from vaccines were made for two main reasons. Firstly, it was to reduce exposure in very small premature babies with low body weight in whom there was a theoretical risk that the intake of mercury from repeated doses of thiomersal-containing vaccines could have been high. Secondly, the intent has been to reduce total exposure to mercury in babies and young children in a world where other environmental sources (particularly in food such as fish) may be more difficult to eliminate. Along with these recommendations, guidelines have been developed on limiting the consumption of certain types of fish, particularly in the diet of pregnant women and young children. This advice is available at http://www.foodstandards.gov.au/whatsinfood/.

In the place of thiomersal, preservatives have either been eliminated from single dose vaccine vials or alternative preservatives have been used. Multidose vaccine vials for are no longer used for routine immunisation in Australia, so the risk of bacterial contamination from withdrawing repeated doses of vaccine is minimal.

What about vaccines for adults?

The levels of mercury in adults resulting from thiomersal-containing vaccines are so low that experts do not recommend removal of thiomersal from vaccines for adolescents or adults. The vaccines available in Australia that currently contain thiomersal are listed below in Table 2.

Which vaccines contain thiomersal?

All vaccines on the current Australian Standard Vaccination Schedule (ASVS) for infants and children under the age of 8 years are now free of thiomersal. The exception is the one of the infant hepatitis B vaccines, Engerix-B paediatric formulation, which contains a greatly reduced amount of thiomersal (2 µg per dose). The following tables list the vaccines used in Australia that are thiomersal free and vaccines that contain thiomersal.

Table 1: Thiomersal-free vaccines available for use in infants and children in Australia

Further reading

Web-based information

1. Mercury and vaccines (Thimerosal). Centers for Disease Control. Link. (accessed November 23, 2004)

2. American Academy of Family Physicians (AAFP), American Academy of Pediatrics (AAP), Advisory Committee on Immunization Practices (ACIP), United States Public Health Service (PHS). Joint Statement concerning removal of thimerosal from vaccines. Link. (accessed November 23, 2004)

3. Centers for Disease Control. Mercury and vaccines Fact Sheet. Link. (accessed November 23, 2004)

4. Recommendations regarding the use of vaccines that contain thimerosal as a preservative. Morbidity and Mortality Weekly Report 1999. Link. (accessed November 23, 2004)

5. Institute of Medicine Immunization Safety Review Committee, Stratton K, Gable A, McCormick MC, editors. 2001. Thimerosal-containing vaccines and neurodevelopmental disorders. Link. (accessed November 23, 2004)

6. Institute of Medicine Immunization Safety Review Committee. Vaccines and Autism. Washington, DC: National Academy Press, 2004, in press Prepublication review available at http://books.nap.edu/catalog/10997.html (accessed November 23, 2004)

7. Institute of Medicine Press Release. MMR Vaccine and Thimerosal-Containing Vaccines Are Not Associated With Autism. May 18, 2004. Link. (accessed November 23, 2004)

8. European Agency for the Evaluation of Medicinal Products (EMEA). Statement on thiomersal in vaccines. Link. (accessed November 23, 2004)

9. Thiomersal and vaccines:questions and answers. World Health Organisation. Scientific papers. Link. (accessed November 23, 2004)

10. Parker SK, Schwartz B, Todd J and Pickering LK. Thimerosal-Containing Vaccines and Autism Spectrum Disorder: A Critical Review of Published Original Data. Pediatrics 2004;114(3):793-804

11. Heron J, Golding J, and ALSPAC Study Team. Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Support a Causal Association. Pediatrics 2004;114(3):577-583

12. Study Fails to Show a Connection Between Thimerosal and Autism. Source: American Academy of Pediatrics, May 16, 2003. Link. (accessed November 23, 2004)

13. Clements CJ. The evidence for the safety of thiomersal in newborn and infant vaccines.Vaccine. 2004 May 7;22(15-16):1854-61

14. Offit, P.A. and Jew R.K. Addressing Parent’s Concerns: Do Vaccines Contain Harmful Preservatives, Adjuvants, Additives, or Residuals?. Pediatrics. 2003 112 (6):1854-1861

15. Madsen KM, Lauritsen MB, Pedersen CB, et al. Thimerosal and the occurrence of autism: negative ecological evidence from Danish population-based data. Pediatrics 2003;112:604-606.

16. Verstraeten T, Davis RL, DeStefano F, Lieu TA, Rhodes PH, Black SB, Shinefield H, and Chen RT. Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases. Pediatrics 2003;112(5): 1039-48.

17. Stehr-Green P, Tull P, Stellfeld M, Mortenson PB, and Simpson D. Autism and thimerosal-containing vaccines: Lack of consistent evidence for an association. American Journal of Preventive Medicine 2003; 25(2): 101-6.

18. Nelson, KB, and Bauman, M.L. Thiomersal and autism? Pediatrics 2003;111:674-679

19. Henderson DC. Mercury in vaccines - reassuring news. Lancet 2002 (Nov 30);360:1711-12.

20. Pichichero ME, Cernichiari E, Lopreiato J, Treanor J. Mercury concentrations and metabolism in infants receiving vaccines containing thiomersal: a descriptive study. Lancet 2002 (Nov 30);360;1737-41.

21. Clements CJ, Ball LK, Ball R, Pratt D. Thiomersal in vaccines. Lancet 2000; 355:1279-1280.

22. Halsey NA. Limiting infant exposure to thimerosal in vaccines and other sources of mercury [editorial]. Journal of the American Medical Association 1999;282:1763-6.

This document was prepared by the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases. Updated November, 2004

This document was written by Associate Professor Raina MacIntyre, Professor Margaret Burgess, Associate Professor Peter McIntyre and Dr Kristine Macartney of the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases.

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Thiomersal Fact Sheet

http://www.ncirs.usyd.edu.au/facts/f-thiomersal.html