I agree that would help make optimal decisions with regards to distributing a limited supply of vaccine doses.
However it's worth noting that even when antibodies drop below levels detectable by standard tests, individuals are likely to retain robust immune responses that provide sufficient protection from severe outcomes [1][2].
[1] SARS-CoV-2 infection induces long-lived bone marrow plasma cells in humans https://www.nature.com/articles/s41586-021-03647-4.pdf
[2] Prior COVID-19 protects against reinfection, even in the absence of detectable antibodies https://www.journalofinfection.com/article/S0163-4453(21)002...
More expensive tests can detect immunity based on T cells for example, which would provide an even longer window.
And of course really expensive and invasive bone marrow aspirate tests could provide a window approaching a lifetime - but the tradeoffs involved there would likely be unfavorable for practical “proof”. Who knows what technology we’ll come up with in the future though.
Ideally, everyone would be tested for antibodies before receiving a vaccine, then we would have data to differentiate naive+vaccine from survivor+vaccine. This would be helpful for both science and public policy.
CDC needs to stop labelling people who are 13 days after their 2nd vaccine dose as "unvaccinated". A more accurate label would be "partly vaccinated".