You're neglecting the time it takes to recruit 40,000 patients. It's not fast or easy to identify those folks, as a significant number of people have disqualif

You're neglecting the time it takes to recruit 40,000 patients. It's not fast or easy to identify those folks, as a significant number of people have disqualifiers (like comorbidities e.g., obesity). Once they're identified as candidates, you still need to go through the task of recruiting them, likely through their healthcare network. You can't just use automated calls for that because nobody's going to trust a recording that promises "Free Vaccine Fast, Act Now!" They likely ran the trial with 40,000 patients because that was all the qualified patients they were able to quickly contact reliably enough to include them in the study (otherwise you have study dropouts, as mentioned in other comments above).