In reply to Ally Doolin of Breckenridge, who asked public health officials why they offered the Pfizer-BioNtech vaccine instead of Comirnaty.

The difference between Comirnaty and the Pfizer-BioNtech vaccine is the exchange of one suspension buffer for another. Buffers are a mixture of salts and sugars that make a liquid medication or IV isotonic (the same salt ratio) to the body. Non-isotonic formulas such as water are painful when injected because the low-salt fluid rushes into cells and bursts them. The body doesn’t distinguish between these isotonic buffers because the different salts achieve the same result: They match the salt ratio in the body. The active ingredient of these vaccines is identical, and testing found the formulas equivalent.

Confusion might also occur because the company also decided to not continue with the name Comirnaty in the new formulation. This is a common occurrence in drug development.