So many people are executed every year in various countries (even the US). Why not allocate them for research purposes?
I'll put my statistician's hat on to answer your fucking stupid scenario.
Are the immune systems of overweight drug-addicted white guys living on a high fat diet directly comparable to the immune systems of subsistence farmers who work a 12-14 hour day and are significantly malnourished, and also have several types of intestinal worms and other chronic infections? (And malaria too, active or passive.)
This is why you test your vaccine (or any other treatment) on a population as closely comparable as possible to your target population.
That leads to another problem with such a testing regime : a significant number of your target population are going to be pregnant or menstruating women, so how are you going to recruit pregnant death-row inmates to your trial? (This is actually a general problem with drug design and testing - there is an understandable reluctance to test drugs for safety and efficacy on pregnant women and unborn foetuses. Breast-feeding women and babies too ; same problem.)
Do people on Slashdot actually try to think through problems before spouting politically-motivated bullshit? Oh, sorry, I just noticed that you're an anonymous coward (with the emphasis on the "coward" part of that).