From Centers for Disease Control and Prevention's website:
Vaccine trial in Thailand
and
more info on the same trial.

This is a very scary thing to test. The only way to ensure effectiveness of a vaccine is to give it to people, then expose them to the virus and see if they catch it. Because they can't do that with HIV, they probably have background statistics on how many new cases there are in a population, and they'll compare rates in the vaccinated folks. Still scary.