I can imagine the ethical approval for this would be difficult, because I think people would be more likely to engage in unsafe sex if they're in the trial. So participating could expose them to more risk than they would otherwise be exposed to.
Do you know how they work around this?
I don't know the answer to your question, but the trials would still be double blind, right? Knowing that I might have been vaccinated would certainly not affect my decision process.
Participating wouldn't be what exposed them to higher risk in the situation you've outlined. This is the sort of strategic wordplay I'd expect from the social sciences.
I think it's a legitimate, if small, concern. There's a possibility that people participating would be more likely to take a roll of the dice. Yes it's stupid, but people do stupid things all the time, particularly when it comes to sex.
Obviously I'm all for the research, I'm just interested to hear if/how they handle it.