South Africa has one of (if not the) highest incidence of HIV in the population, so it makes sense to test it there. If you test it in a country where the incid

South Africa has one of (if not the) highest incidence of HIV in the population, so it makes sense to test it there. If you test it in a country where the incidence is 0.01 of the population, it's very hard to see statistical significance. Edit: to add to my comment, it's also a phase 3 trial, so the safety has already been proven before.