Vaccines contain many ingredients that make them safer and more effective, including preservatives, adjuvants, and other ingredients. Click on the categories below to jump to information on that ingredient.




Thimerosal is a mercury-containing preservative used in some vaccines and other products since the 1930s. It is very effective in preventing bacterial contamination of multi-dose vials. Some people believed that the mercury in thimerosal was harmful. Much of this belief came from studies of methylmercury, which is found in fish and the environment and is toxic in high doses. However, thimerosal contains ethylmercury which is broken down and leaves the body much more quickly.


In July 1999, the Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure. We can't always remove mercury from the environment, but we could control the mercury used in some vaccines. So by taking thimerosal out of vaccines, we lessened the amount of mercury a child was exposed to early in life.

Since that time, studies have found that there is no convincing scientific evidence that the low dose of thimerosal in vaccines causes harm, except for minor reactions like redness and swelling at the injection site. Since 2001, with the exception of some influenza (flu) vaccines, thimerosal is not used as a preservative in routinely recommended childhood vaccines, yet autism rates have not gone down.


For More Information on Thimerosal

Research Studies

  • AAP Listing of Studies Examining Thimerosal and Autism (PDF 98KB)

  • Immunization Safety Review: Vaccines and Autism. Institute of Medicine, The National Academies Press: 2004

  • Infant and Environmental Exposures to Thimerosal and Neuropsychological Outcomes at Ages 7 to 10 Years. Thompson, et al. 2007, New England Journal of Medicine. 357:1281-1292

  • Autism and Thimerosal-Containing Vaccines: Lack of Consistent Evidence for an Association. Stehr-Green, P, American Journal of Preventive Medicine 2003; 25:101-106.

  • Continuing increases in autism reported to California's developmental services system: mercury in retrograde. Schechter and Grether, 2008, Archives of General Psychiatry. 65(1):19-24

  • Mercury Levels in Newborns and Infants After Receipt of Thimerosal-Containing Vaccines Pichichero, et al., Pediatrics. Vol. 121 No. 2, 2008, pp. e208-e214

  • Nelson KB, Bauman ML. Thimerosal and autism? Pediatrics. 2003;111:674-679



Aluminum salts help your body create a better immune response to vaccines. Aluminum salts are necessary to make some of the vaccines we use more effective. Without an adjuvant like aluminum, people could need more doses of shots to be protected. Everyone is exposed to aluminum because there is much aluminum in the earth's crust. It's present in our food, air, and water, even in breast milk and formula. The amount of aluminum in vaccines is similar to that found in 33 ounces of infant formula. Aluminum has been used and studied in vaccines for 75 years and is safe.


For More Information on Aluminum


Research Studies

  • Baylor NW, Egan W, Richman P. Aluminum salts in vaccines—U.S. perspective. Vaccine 2002; 20: S18-S23.


Other Ingredients

Vaccines contain a variety of ingredients that help them be more effective. Some people say that vaccines contain antifreeze, which is not true. Frequently, parents also wonder about formaldehyde, aborted fetuses, gelatin, and other ingredients. Information below addresses each of those issues.


For More Information on Ingredients


Research Studies


Last Updated: 4/16/2013


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