Remember that Phase 3 trials measured vaccine effectiveness v placebo – assuming same behavior. In real life, most people relax their behavior after taking a vaccine, like a good ice cream after a diet …
And when you now have a mass of people who congregate together with the vaccine, then they used zechuyot from the vaccine to reduce distance between themselves rather than to be safer.
Ironically, masks seem to lead to higher SD: people see masks and then behave accordingly. But when people see vaccinated people, they do not know vaccination status, leaading to lower SD.
This is a good illustration of Jewish symbolism that reminds us of proper behavior – mezuzah, tzitzis, shtreimel … on the opposite side, this is like good/bad neighbor from pirkei avos – your risk depends on the neighbors.