(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'.

Vaccine trials are essentially this. You give some percentage of the population the vaccine, and you compare their survival rates to the rest of the population.

Your control group during the trial gets placebos, and you compare how many contracted the thing you're trying to prevent in two groups.

In some ways simpler than the control group getting the best current treatment, in which case you have to compare the efficacy in both groups.

Presumably you would have to do a longitudinal study (could be an RCT or cohort or something else). If I think about it, it seems like studying how long a vaccine is effective for is a similar problem.

Give a bunch of people the drug and a bunch of people a placebo, wait a long time (decade+), see if there is a statistically-significant decrease in the thing that it is trying to prevent.