The same way how if you get a scan after chemo or surgery or radiation and see no cancer at 3 month follow up you don’t declare “you’re cured.”
Longer follow up is important in clinical trials.
Hard to know how much efficacy was lost with time or with viral mutations though.
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However, with a novel pandemic/virus you don’t have the luxury for more time. But a lot of smart people will tell you to run the trials, get adequate follow up, so you have good data on the back end.
A big part of the Covid response is tailoring messaging to prioritize promotion of a behavior over prioritizing discussing the actual data/truth. Kind of integrating Cass Sunsteins Nudge philosophy into cdc messaging. I think it works in short term (may promote more vax /mask uptake), but the tail risk is enormous if you mess up public trust in institutions.