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Thimerosal in Flu Vaccine

Questions & Answers

What is thimerosal?

Thimerosal is an ethyl mercury-based preservative used in vials that contain more than one dose of a vaccine (multi-dose vials) to prevent germs, bacteria and/or fungi from contaminating the vaccine.

Why are preservatives sometimes used in vaccines?

Preservatives are used to protect 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. Preservatives are needed to prevent contamination of multi-dose vials each time individual doses are drawn.

Do flu vaccines contain thimerosal?

Flu vaccines in multi-dose vials contain thimerosal to safeguard against contamination of the vial. Most single-dose vials and pre-filled syringes of flu shot and the nasal spray flu vaccine do not contain a preservative because they are intended to be only used once.

A list of available flu vaccines and their thimerosal content is available.

Where can I learn more about thimerosal?

Please visit CDC’s webpage Thimerosal in Vaccines for more information on thimerosal, including Frequently Asked Questions about Thimerosal.

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