What’s in vaccines?
There are three main types of ingredients that may be found in vaccines:
Active Ingredients
The active ingredients in vaccines are the parts of the viruses or bacteria that are used to teach your immune system how to defend itself against a particular disease.
Vaccines may also contain a small amount of the material used to grow the vaccine virus or bacteria (eg, chicken-egg protein).
Preservatives/Stabilizers
Preservatives help maintain a vaccine’s purity. Small amounts of preservatives help prevent unwanted bacteria from contaminating a vaccine.
Stabilizers may be added to help ensure the vaccine stays unchanged after contact with heat, light, etc.
Adjuvants/Enhancers
Small amounts of adjuvants [AD-‘joov-‘ants] may be added to a vaccine to help boost its effectiveness.
Ingredients that may be found in vaccines
Aluminum
Aluminum salts are used in some vaccines to help enhance the immune response in the person who receives the vaccine.
Mercury/Thimerosal
Thimerosal is a mercury-containing compound that has been used since the 1930s to help prevent bacteria from contaminating vaccines.
Thimerosal has been removed or reduced to trace amounts in most vaccines recommended for children aged 6 and younger.
Antibiotics
Some antibiotics may be used during the manufacturing of vaccines to help prevent contamination from bacteria.
Very small amounts of these antibiotics may be in the vaccines themselves.
Formaldehyde
Formaldehyde has been used for many years to inactivate viruses and bacteria during the vaccine manufacturing process.
Very small amounts of formaldehyde may be found in some vaccines.

The CDC, FDA, NIH, and other federal agencies routinely monitor vaccines and conduct research to assess their safety.

Questions about vaccination
Common questions about vaccination
CDC = Centers for Disease Control and Prevention;
FDA = Food and Drug Administration; NIH = National Institutes of Health.

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