The real problem I see is that we count vaccine doses as one shot. For Pfizer and Moderna two shots = one dose. It is misleading to show a two vial dose as two doses.
That’s the benefit.
Note: this was always accounted for as “extra” doses. It was part of the plan that there would be extra volume for each vial that could be pooled together for extra full doses. But because they are “extra”, you couldn’t plan on there always being that extra dose (or over the course of a day how many extra doses).
Initially some doses were wasted because it was unclear whether that was a legitimate thing to do.
Then, after explicit guidance, they said "OK, these count as 6 doses, then, for the purpose of our Federal contract!"
Then, Feds scrambled to ship supplies to ensure low-dead-space syringes were being used everywhere.
That feels like one of the points where bureaucracy is stupidly killing people.
Having bad outcomes from pooling and not knowing whether it came from the combining practice, a certain vial, an entire lot, or even worse-- the fundamental vaccine-- could take the entire vaccination program off the rails. That's not worth it to stretch supplies a few percent.