Preservatives are used in some vaccines to prevent bacterial or fungal contamination. The requirement for preservatives in vaccines arose from many incidents in the early 20th century of children who developed severe and occasionally fatal bacterial infections after administration of vaccines contained in multi-dose vials. For example, in 1916, four children died, 26 developed local abscesses, and 68 developed severe systemic infections after receipt of a typhoid vaccine contaminated with the bacteria Staphylococcus aureus. As a consequence of this and similar incidents, preservatives have been required for vaccines contained in multi-dose vials (with some exceptions) since the 1930s.
Thimerosal, a mercury-containing preservative, has been the focus of intense scrutiny by the U.S. Congress and the news media. Thimerosal is no longer used as a preservative in any childhood vaccine with the exception of the influenza vaccine. Attention by the news media has caused some parents to fear that thimerosal contained in vaccines might have harmed their children.
Removal of thimerosal from vaccines was precipitated by an amendment to the Food and Drug Administration (FDA) Modernization Act that was signed into law on November 21, 1997. The amendment gave the FDA two years to "compile a list of drugs and foods that contain intentionally introduced mercury compounds and ...[to] provide a quantitative and qualitative analysis of the mercury compounds in the list...." The amendment arose from a long-standing interest in lessening human exposure to mercury, a known neurotoxin (a protein that harms the nervous system) and nephrotoxin (a protein that harms the kidneys).
At the time the FDA Modernization Act was passed, infants were recommended to receive three different vaccines that contained thimerosal — diphtheria-tetanus-acellular pertussis (DTaP), hepatitis B and Haemophilus influenzae type b (Hib). Infants receiving all of these vaccines could have been exposed to a cumulative dose of mercury as high as 187.5 micrograms (ug)* by 6 months of age. The cumulative dose exceeded guidelines recommended by the Environmental Protection Agency (EPA) (see table below). Thimerosal, as a preservative, is no longer contained in any childhood vaccine, with the exception of the influenza vaccine.
*Note: One microgram is one-millionth of a gram. One gram is the weight of one-fifth of a teaspoon of water.
|EPA||65 ug||89 ug||106 ug|
|ATSDR||194 ug||266 ug||319 ug|
|FDA||259 ug||354 ug||425 ug|
|Exposure limits for mercury in infants less than or equal to 6 months of age by percentile body weight established by the Environmental Protection Agency (EPA), the Agency for Toxic Substance Disease Registry (ATSDR), and the Food and Drug Administration (FDA)|
Mercury is a naturally occurring element found in the earth's crust, air, soil and water. Since the earth's formation, volcanic eruptions, weathering of rocks and burning of coal have caused mercury to be released into the environment. Once released, certain types of bacteria in the environment can change mercury to methylmercury. Methylmercury makes its way through the food chain in fish, animals and humans. At high levels, it can be toxic to people.
Thimerosal contains a different form of mercury called ethylmercury. Studies comparing ethylmercury and methylmercury suggest that they are processed differently in the human body. Ethylmercury is broken down and excreted much more rapidly than methylmercury. Therefore, ethylmercury (the type of mercury in the influenza vaccine) is much less likely than methylmercury (the type of mercury in the environment) to accumulate in the body and cause harm.
Because the names of these two chemicals vary by only a single letter, it is difficult to believe they could be very different; however, if you think about ethanol and methanol (also known as ethyl alcohol and methyl alcohol), you will see that the difference can be dramatic – ethanol is what we drink at a party and methanol is added to the gasoline we use to fuel our cars.
Despite concerns, several studies have now proven that thimerosal in vaccines did not cause autism:
In addition, several other pieces of information add to the reassurance provided by these studies:
A. Hviid, et al., “Association Between Thimerosal-Containing Vaccine and Autism,” Journal of the American Medical Association 2003; 290:1763-1766.
T. Verstraeten, et al., “Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases,” Pediatrics 2003; 112:1039-1048.
J. Heron, J. Golding, and ALSPAC Study Team. “Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Show A Causal Association,” Pediatrics. 2004; 114:577-583.
N. Andrews, et al., “Thimerosal Exposure in Infants and Developmental Disorders: A Retrospective Cohort Study in the United Kingdom Does Not Show A Causal Association,” Pediatrics, 2004; 114:584-591.
Fombonne, E., et. al. "Pervasive Developmental Disorders in Montreal, Quebec, Canada: Prevalence and Links with Immunizations," Pediatrics. 2006; 118:139-150.
Chess S, Fernandez P, Korn S. “Behavioral Consequences of Congenital Rubella,” J Pediatr, 1978; 93:699-703.
Deykin EY, MacMahon B. “Viral Exposure and Autism,” Am J Epidemiol 1979; 109:628-638.
Rodier PM. “The Early Origins of Autism,” Scientific American February 2000, pp.56-63.
Stomland K, Nordin V, Miller M, et. al. “Autism in Thalidomide Embryopathy: A Population Study,” Developmental Med Child Neurol 1994; 36:351-356.
Nelson KB, Bauman ML. “Thimerosal and autism?” Pediatrics 2003:111:674-679.
Gundacker C, Pietschnig B, Wittmann KJ, et al. “Lead and Mercury in Breast Milk,” Pediatrics 2002; 110:873-878.
Pichichero ME, Cernichiari E, Lopreiato J, Treanor J. “Mercury Concentrations and Metabolism in Infants Receiving Vaccines Containing Thimerosal: A Descriptive Study,” Lancet 2002; 360:1737-1741.
Stehr-Green, P, “Autism and Thimerosal-Containing Vaccines: Lack of Consistent Evidence for an Association,” Am J Prev Med 2003; 25:101-106.
Madsen, K. “Thimerosal and Occurrence of Autism: Negative Ecological Evidence from Danish Population-Based Data,” Pediatrics 2003; 112:604-606.
Thomson, B, Price, C. et. al. “Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years,“New England J of Med 2007; 357:1281-1292.
Reviewed by: Paul A. Offit, MD
Date: March 2014
Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.