By waiting for a large number of infections to develop, then unblinding the test and seeing how many were in the placebo cohort and how many received the actual vaccine. That’s the procedure used by all major vaccine candidates, and the results have been pretty uniform for Pfizer and Moderna so far.
While your way would have been significantly faster, ethical concerns prevented them from conducting a trial like that.
All good, however we know so little about transmission and the data keeps on changing, with these people in the trial likely not eating out in restaurants, wearing masks and practicing social distancing, what does it tell us when one group has 90 infected and the other 11? isn't it scientifically plausible that the results could've been much closer?
If my goal is to establish the efficacy of the vaccine in immunizing the patient against being infected "when exposed" what does the trial tell me?