While your point is well taken given historical context of rampant risky medical trials conducted against african and african american populations that are pret

While your point is well taken given historical context of rampant risky medical trials conducted against african and african american populations that are pretty horrifying, I suspect it has more to do with the high concentration of HIV per capita in Africa. To my knowledge, Africa had one of the highest HIV rates in the world so I suspect this is still the case making it a very reasonable region for the trials. If you had an "obesity" vaccine, the US would be a good population to do trials vs say Ethiopia or Sri Lanka. If you have a Dengue vaccine, you probably wouldn't want trials in northern Canada but would probably target southeast Asia instead.