Looks like the US has passed a threshold of sorts:
27.5M Americans vaccinated > 26.5M confirmed infected.
Certainly there's more than 26.5M who were infected as that's only those who have tested positive. Minus the 500K who shall remain Covid dead. Assuming those infected now have natural immunity, and 27.5M have been vaccinated, the US might have around 60M or 70M or more who have degrees of resistance. This could be why new cases are dropping. While the vaccinated folks are high risk groups such as the elderly, sick, medical workers.
Of course the goal is herd immunity and to end transmissions, but once you vaccinate a good deal of the most vulnerable, transmissions, hospitalizations, deaths all should drop significantly. After 3 months of vaccinations, things should be fairly well contained. The virus appears seasonal, so infections should naturally be dropping by July and August, when you should have well over 200M Americans vaccinated by July 4. Still early days and things can still go wrong (mutations, running out of ingredients, vaccines not conferring immunity for very long, etc), but there's certainly light at the end of the tunnel, and a post-virus world in sight.