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What Is Tromethamine, or Tris? It Helps Stabilize Covid Vaccines


When the Food and Drug Administration authorized the emergency use of the Pfizer-BioNTech vaccine in children ages 5 to 11 last month, the agency noted that the pediatric version would be a bit different than the one for adults: a third the size, with a different buffer for added stability.

The news set off a wave of disinformation on social media about the new ingredient: Tris, or tromethamine.

The ingredient, however, has a time-tested track record of safety. It is “a commonly used buffer in a variety of other F.D.A.-approved vaccines and other biologics, including products for use in children,” the agency said in its statement announcing the authorization for Pfizer’s pediatric doses.

Such buffers “help maintain a vaccine’s pH (a measure of how acidic or alkaline a solution is) and stability,” according to the F.D.A., which says that the new formulation gives vaccine providers more flexibility in storage.

Pfizer’s pediatric dose also removes extra salt — sodium chloride and potassium chloride — according to Dr. Sandra Fryhofer, the American Medical Association’s liaison to a Centers for Disease Control and Prevention immunization committee. Taken together with the “tris buffer,” she said in an A.M.A. interview, the changes “make the vaccine product more stable at regular refrigerator temperatures for longer periods of time. The kid version vials can be stored unopened in regular refrigerators for up to 10 weeks.”

The ingredient is not specific to Pfizer’s pediatric Covid vaccine doses. Kit Longley, a Pfizer spokesman, said in an email on Saturday that the compound was being used in adult doses as of this month, and that the manufacturing and ingredient list were otherwise unchanged. Tromethamine is also used in Moderna’s Covid vaccine.

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