Agree that vaccine immunity is dependent on many factors. Regardless, we must measure amount of antibodies to determine the individual's immunity over time and when that individual will need next booster shot. I am only speculating that a future commercial common antibody test will be able to measure amount of antibodies in the blood but that would answer a lot of questions. That would be a great DIY Home test for those already vaccinated.

In the meantime, antibody tests show if you have antibodies or not. Still useful for determining when to have booster shot.
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If it is found that vaccine does not stop reinfection at nasal/throat area then regular Antigen tests still needed.