Exactly. In this case, we're lucky enough to use an amazing clinic set up in Durban, South Africa, which has a ~15% prevalence rate of HIV.
It seems like believing you are vaccinated would increase risky behaviour. This is bad in itself. It is also bad if the vaccine is only partially effective, since the risk effect might balance the vaccine effect.
With a placebo, knowing you only have a 50/50 chance of getting the actual trial vaccine might also help mitigate the risk effect.
Anyway, just wondering how all this works. Finding an effective vaccine would be fantastic; keep up the good work!
But this will also be true of the vaccine as employed in reality, and thus a useful piece of information to capture.