Does my COVID-19 Antibody Test Results Provide me with any Actionable Data?

I have donated blood six times since the start of the pandemic. I don’t know when they started testing for COVID antibodies, but in July of 2020 I got my first result, which was negative. To me; that was useful information, in that it showed that the precautions I was currently following were sufficient to avoid exposure.

I got another negative test in December; so far so good. I was vaccinated in January of 2021 with the Moderna vaccine with the second dose coming in February, then my late February donation test results showed reactive, which is what they say should be the result of a vaccinated person who has not had COVID.

Since then I donated in April and June and both tests were still reactive. Does a reactive test indicate my immunity is still in effect? I’m a little concerned that some data indicate the mRNA vaccine efficacy tends to diminish more quickly than one would desire. Am I allowed to quit worrying about immunity as long as I remain reactive?

Also, if I caught one of the asymptomatic cases of breakthrough COVID would my antibody test switch from reactive to positive, indicating that I needed to warn my close contacts? Although vaccinated I have people in my bubble who aren’t so I’d have to give them a heads up to be tested.

Antibodies disappear over time, there are B and T cells that give lasting immunity. This article from Nature covers it, but it is from last August so understanding may have changed.

Not just antibodies: B cells and T cells mediate immunity to COVID-19 | Nature Reviews Immunology