Really? You can give them a placebo and say, "This might reduce your risk of HIV or it might not. Have fun!"
I'm curious because my impression is that most drug trials have been for AIDS treatments (where, perhaps, placebos are more ethically acceptable-- especially before the treatment is known to be better than the placebo) rather than HIV prevention.
I'm not an expert on the ethics, but pretty much every vaccine trial faces this dilemma. The solution is to make sure that the trial participants are fully informed (i.e. they've got to know that they may not be getting a real vaccine), and to make sure that the vaccine in question doesn't make things worse.
If you can do those two things, then in the worst case, the participants are no worse off than if they hadn't participated in the trial at all. But as I said, clinical trials for things like HIV vaccines tend to go above and beyond, and do things like safe-sex counseling for every participant.
I'm curious because my impression is that most drug trials have been for AIDS treatments (where, perhaps, placebos are more ethically acceptable-- especially before the treatment is known to be better than the placebo) rather than HIV prevention.