But if you just do the trial you don't know if you actually got the vaccine or placebo.
I mean, take the flu shot. The times I've gotten it I've not had any reaction or one so mild I don't remember. Maybe a sore arm? Slightly tired, though that could have been work. I wouldn't personally know until they do blood tests.
Even if you have side effects - outside of a possible sore arm, you wouldn't know it was the vaccine or just unfortunate timing for a virus or something.
You want the trial to be double blind because it means there is less bias: https://www.verywellmind.com/what-is-a-double-blind-study-27....
I don't know what the poster above was talking about, getting calls to participate in a study. Nobody contacted my wife or I with such offers. I suspect that the poster was lucky to live right in a city where the test was being conducted; those outside of major cities are probably not included in his claim that anyone could participate in the trials.
Sometimes it just doesn't work to have double blind: LSD trials, for example, make things pretty obvious. It isn't the case for the flue, though.
You are correct on the trials: most folks don't get called. That said, living outside the city doesn't exclude you from trials in general because they generally need participants with x and/or y. Often, they will pay for expenses to travel, especially if your disease is uncommon or they have trouble finding participants.