““The way to get the answer is to just follow people closely enough to determine when that level of efficacy or protection diminishes, both with regard to the level of the antibodies as well as clinical data with regard to breakthrough infections,” Fauci said. (Breakthrough cases refer to when people who are vaccinated get infected.)”
CDC has reported 5,800 breakthrough cases so far.
Just remember, an unknown (to us) subset of this 5800 is nothing more than a "positive PCR test". There are probably 10's of thousands of other people who would test positive, but have no reason to be tested. It would be much more valuable to know how many people actually had a positive PCR AND symptoms. Just as before, "cases" don't matter. Hospitalizations and deaths do.