Thimerosal is safe. It’s only used in some multi-dose flu shots and not in routine childhood vaccines.
This week the newly appointed vaccine advisors to the CDC are expected to meet to vote on “recommendations regarding thimerosal containing flu vaccines.” [archived link]
Thimerosal, also known as ethyl mercury [archived link], is a harmless preservative found in many multi-dose flu vaccine preparations. It is used to keep bacteria or fungi from growing in a vaccine vial that is accessed more than once. Most manufacturers have moved towards single-use vials, which not only reduce vaccine wastage but also to get rid of the need for preservatives like thimerosal.
Thimerosal is no longer used in childhood vaccines. It is still used in some flu vaccines, but only those that contain more than one dose. It can also be found in tiny amounts in the adult/teen Tetanus (Td, NOT Tdap) shots. This small amount is due to the way the shot is made (and not meant to act as a preservative.) [archived link] There is no thimerosal in COVID-19 vaccines or Fluarix (a flu shot specifically for kids.)
Thimerosal has gotten a bad rap partly because of its similarity in name (remember it’s known as ethyl mercury) to methyl mercury, which is incredibly toxic. Ethyl and methyl groups differ by only 2 hydrogens and a carbon, but these little letters make a big difference. Ethyl mercury AKA Thimerosal is not toxic at the levels present in these vaccines, and is quickly broken down and removed by the body, while methyl mercury is bad news – it can build up in tissues and in certain doses, it can cause damage to the nervous system and kidneys.
Resources:
What’s in childhood vaccines? — Those Nerdy Girls
CDC vaccine advisers to vote on thimerosal in flu shots at first meeting of new panel | CNN
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