Up to now, people have been discouraged from getting antibody tests to measure what their COVID vaccination did. The reasoning behind that advice is that since nobody knew what a given level meant in actionable terms, it was not useful information.
That just changed.
Nature has published the results of research into the relationship between antibody levels post-vaccination and vaccine efficacy, with plenty of different vaccines.
An overview, easy to read, is at
The full paper, with the methodology and conclusions, is at
It looks like careful work but it's over my head in places.
The bottom line is that they found a clear correlation and protection levels keep going up as antibody levels go up, though there's some diminishing returns at the very top end where the mRNA vaccines are.
The cool part is they figured out what antibody level would give you the 50% protection that we would have been willing to accept from a vaccine. The mRNA vaccines leave you with an order of magnitude more. That's quite a comforting safety factor against variants and gradual decline.