If Alice has covid, and coughs on Bob, who is vaccinated, there will likely be some covid infected droplets get into Bob's respiratory system and his sinus. The virus may live inside his body for a period of time, and may even reproduce while inside his body without the 'assistance' of his body. If Bob is tested later that day, he may show as being infected, even if the test picks up Alice's droplets. I don't know that this has ever happened, or if it does happen, how frequently it happens. At no point would Bob's body act any differently than it would otherwise act.
If the above happens, is it really accurate to say that Bob is infected?
If this happens, then perhaps not. And if, because of this, the chance of infection amongst vaccinated people is actually lower than believed, then it strengthens the case for vaccination.
If Alice has covid, and coughs on Bob
The other issue with this is that
Peter would likely be quite displeased at Alice's relationship with Bob, given her past history.
I'm not consciously migrating our discussion to a different thread with each post I make; it's just how it's working out.