Meryl Nass

On Wednesday, August 31, the FDA issued emergency use authorizations for new Pfizer and Moderna mRNA booster vaccines for COVID.  The next day, September 1, the CDC’s advisory committee and CDC Director approved the immediate rollout of the new vaccines. They will be administered in the US starting this week.

Surprisingly, more than a month before either agency had given its okay to the entirely new formulation, the federal government ordered 105 million doses from Pfizer and 66 million doses from Moderna.

The desired composition of the vaccine had only been formally determined by FDA after its advisory committee had met on June 24, 2022. The vaccines contain a mix of the old, original Wuhan strain vaccine mRNA (now also referred to as the ancestral vaccine) and a new omicron BA.4/5 mRNA coding for the omicron spike protein. 

The total amount of mRNA for the Pfizer and Moderna booster vaccines is the same as before:  30 mcg for Pfizer and 50 mcg for Moderna.  Each is composed of 50% omicron mRNA and 50% ancestral mRNA, and they are termed bivalent vaccines.  The new vials and their boxes do not list the dose, hinting that the decision regarding how much to use was made very recently.  Even the members of the CDC’s advisory committee did not know the dosage of the new bivalent vaccines until their September 1 meeting.