(While we wait for expert views) I would say it was no different from any other medical trial (you would have two populations, one getting a treatment and one not getting it, and you would observe them and see if there was a statistically difference) - With a preventative drug you would be looking for people getting cancer - this is a rare event (and so need more people under observation) and would take many years before you had a marked difference in the two populations.

Compare this with late stage cancer trial where you are monitoring people dying - this is going to, unfortunately for the people under observation, give you a much quicker 'outcome'.