Interestingly, it's probably not those in Medicine who make these assessments and drive the decisions. That would be the pure scientists; the biologists and virologists along with actuaries and statisticians. To me those in Medicine (or practicing medicine) are the applied scientists (Physicians, Sugeons, NP's).
There isn't enough data to actually KNOW what is going to happen - these vaccines haven't been around long enough to know. But we put our trust in the consensus thinking of the scientific community to know that it is probably the best path. (Most interestingly, it is probably the politicians who will get skewered if they are wrong,

but I digress and run close to the set boundary).
So sure, we might need a 3rd shot if they are wrong, but I am willing to get a 3rd shot later if thousands or more are saved. Actually, we don't even know yet how long the recommended 2-shot path of vaccination will last. There is a very good chance we could be rolling up our sleeves every fall from now on to get our covi-flu injections.