Q: What other ingredients do vaccines contain?

A: In addition to their active components, vaccines often contain other ingredients that are in the vaccine for a specific reason. These other ingredients are usually present in tiny amounts and are not harmful.

You may need to know about the other ingredients in case of an allergy, food intolerance, or religious or cultural dietary limitations.

Serious vaccine side effects are very unlikely. In rare cases, they may cause allergic reactions or anaphylaxis (a serious form of allergic reaction that happens rapidly). If you have any allergies, or if you have had a bad reaction to any vaccine or medicine in the past, tell your doctor.

Other ingredients in vaccines include:

  • Preservatives to make sure the vaccine does not become contaminated (not present in all vaccines).
  • Additives to help keep the ingredients in the vaccine stable when it is stored, so that it stays active and effective.
  • Adjuvants — other ingredients that help the vaccine to work.
  • Liquid to dilute the vaccine to the right dose.
  • Tiny amounts of other ingredients from the production process. For example, the flu vaccine may contain traces of egg protein.

For more information read this factsheet about vaccine components.