At first glance this looks horrifying, suggesting that the test vaccine gave the group AIDS, although I don't believe that is what happened.
I'm no expert, but this is how I believe vaccine studies work:
-Researchers give volunteers dummy shots and tell the volunteers to live his or her life as they normally would. This, for some people, means screwing around without protection, sharing needles or however the person exposes themselves to the virus, and contracting a disease because of that. And researchers know this. Hence the 21 new cases of HIV out of the 762 dummy shot volunteers (these people don't know they are getting a dummy shot though.)
-Researchers also give the trial vaccine to a similar size group of volunteers, and once again, tell them to live their life as they normally would, knowing that a small percentage of those people will expose themselves to the HIV virus.
-Afterwards, the compare the two groups to see if the vaccine had statistically significant effect of preventing contraction of HIV. In this case, it did not.
Its really the only way to test the possible effectiveness of early vaccines without deliberately giving people the virus and "hoping for the best."
Unless I'm totally off base, and if I am then I just made a fool of myself.