What’s in the flu vaccine?

Vaccines help develop immunity by imitating an infection. This doesn’t cause illness, but it does cause the immune system to produce antibodies that help protect you from the disease in the future. Vaccines contain ingredients called antigens, which tell the body’s immune system to create antibodies to protect against those antigens.

Every day, a healthy child’s immune system successfully fights off thousands of antigens — the parts of germs that cause the body’s immune system to go to work.

The antigens in vaccines come from the germs themselves, but the germs are weakened or killed so they cannot cause serious illness. Vaccines contain only a tiny fraction of the antigens that children encounter every day in their environment, even if they receive several vaccines on one day.

Ingredients in vaccines are all there for a reason — to help the vaccine work better and to guarantee vaccine safety. Most ingredients in vaccines are already present in our bodies or our environment — the air we breathe, the water we drink and the food we eat.

Common ingredients in vaccines are:

  • Adjuvants: Adjuvants (like aluminum) help vaccines work better. They enhance the immune response, decreasing the quantity of vaccine needed to gain protective immunity, or lowering the number of doses required.
  • By-products: Some products (like formaldehyde) are used during vaccine manufacturing to make sure viruses are inactivated and are removed except for a tiny trace.
  • Stabilizers: Stabilizers (like gelatin) are added to vaccines to protect the active ingredients from breaking down during manufacture, transport and storage.
  • Preservatives: Preservatives (like trace amounts of thimerosal) prevent bacterial or fungal contamination. Early in the 20th century, most vaccines were packaged in vials that contained multiple doses. Doctors and nurses would draw up a single dose and place the vaccine back in the refrigerator. Unfortunately, sometimes bacteria would inadvertently enter the vial and cause infections. Preservatives, originally added in the 1930s, solved this problem.

Thimerosal, a mercury-based preservative, is used in trace amounts in some versions of the flu vaccine, although most versions are thimerosal-free. This preservative is used to protect flu vaccines packaged in multi-dose vials. Each time a vaccine dose is drawn from a multi-dose vial, bacteria or fungi can enter the vial. Receiving a vaccine contaminated with bacteria or fungi can be dangerous and cause infections. Preservatives help prevent contamination of multi-dose vials each time individual doses are drawn.

Mercury is a naturally occurring element found in the earth’s crust, air, soil and water. As a result, we are all exposed to mercury. Today, breastfed infants are exposed to 15 times more mercury in breast milk than is contained in the influenza vaccine.

More information on thimerosal is available from the Centers for Disease Control and Prevention (CDC) and the Children’s Hospital of Philadelphia. A  is available from the CDC.

Other ingredients: The amount of good and bad information online about vaccines can be overwhelming and confusing, but you can’t believe everything you read about ingredients in vaccines. For example, no vaccine contains, or has ever contained, antifreeze.

It’s important to look to the most trusted physicians, research, and public health organizations in the world for answers. The CDC has compiled an ingredient list for most vaccines. While this information may be helpful for people concerned about particular allergies or ingredients, it is important to realize that the ingredient list does not indicate quantities in each vaccine. In most cases, the quantities are so small that they do not cause allergic reactions or symptoms of toxicity. Talk to your child’s doctor about any questions or concerns you have about vaccine ingredients.

CDC: Ingredients of Vaccines Fact Sheet
CDC: Parents’ Guide to Childhood Immunizations FAQ
Children’s Hospital of Philadelphia: Vaccine Ingredients Q&A
CDC: Thimerosal in Flu Vaccine
CDC: Flu Vaccine Safety Information


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As parents, determining how best to protect our children can be overwhelming and confusing. We’re here to help.

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About I Vaccinate

I Vaccinate provides information and tools based on real medical science and research to help Michigan parents protect their kids. Support is provided by the Michigan Department of Health and Human Services and the Franny Strong Foundation.

You’ve got questions. That’s a good thing.

As parents, determining how best to protect our children can be overwhelming and confusing. We’re here to help.

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