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In easy-to-understand language
Note: The terms "immunization," "vaccination," and "inoculation" are used to mean essentially the same thing throughout this site.
Brief Overview
  • Millions of doses of vaccines are administered to children in this country each year. Ensuring that those vaccines are potent, sterile, and safe may require the addition of minute amounts of additives.
  • Substances are added to vaccines to enhance the immune response, to prevent microbial contamination, and to stabilize the vaccine formulation.
  • The amount of additives found in vaccines is usually small.
Additives Found in Vaccines
Additives used in the production of vaccines may include:
  • Preservatives - such as thimerosal or 2-phenoxy ethanol, are added to slow or stop the growth of bacteria or fungi resulting from inadvertent contamination, especially as might occur with vaccine vials intended for multiple uses or doses.
  • Stabilizers - such as lactose or monosodium glutamate (MSG), are added to stabilize the vaccine formulation against a variety of conditions, such as temperature variations or a freeze-drying process.
  • Adjuvants - such as aluminum hydroxide or aluminum phosphate, are added to increase the ability of the vaccine to trigger, enhance, or prolong an immune response.
  • Antibiotics - such as neomycin and streptomycin, are added to prevent the potentially harmful growth of germs.
  • Other Substances - Vaccines may also include a suspending fluid such as sterile water or saline. Vaccines may also contain small amounts of residual materials from the manufacturing process, such as cell or bacterial proteins, egg proteins (from vaccines that are produced in eggs), DNA or RNA, formaldehyde from a toxoiding process, etc; while these materials are not "additives" per se, they may nonetheless be present in vaccine formulations.
Vaccine Safety
  • All packaging for vaccines includes a package insert, which lists all ingredients in the vaccine and discusses any known adverse reactions. To find out what additives are in specific vaccines, ask your health care provider or pharmacist for a copy of package insert.
  • Persons with a prior history of allergic reactions to any of the substances in a specific vaccine should consult their health care provider before vaccination.
  • To assure the safety of vaccines, the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and other Federal agencies routinely monitor and conduct research to examine any new evidence that would suggest possible problems with the safety of vaccines.
  • To report a health problem that followed vaccination you or your provider should call the Vaccine Adverse Event Reporting System (VAERS) at (800) 822-7967.
  • All of the vaccines that are routinely recommended for infants, including hepatitis B, haemophilus influenzae type b, DTaP, pneumococcal conjugate, IPV, MMR, and varicella are available in formulations without thimerosal as a preservative.
CDC, National Immunization Program:

FDA, Center for Biologics Evaluation and Research:

Last updated: June 2001

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Centers for Disease Control and Prevention
National Vaccine Program Office