I'm in a phase 3 study in Canada for a not-yet-approved vaccine called Medicago. It was a crossover study where you would either get the vaccine now or later, but there were some administrative issues with the second half of it. Between the delays associated with that and the sooner-than-expected availability of the mRNA vaccines in Canada, they ended up agreeing to unblind everyone with the thought that anyone who was in the placebo-first group would drop out and get an approved vaccine.

Those of us who got that vaccine first were encouraged to remain in, though, which I did. Though that may change depending on mandate/passport stuff, since for official purposes, I'm still considered unvaccinated. There have been a number of other issues like this that have come up, for example:

https://www.thestar.com/news/canada/2021/08/13/vaccine-passp...

https://www.cbc.ca/news/canada/newfoundland-labrador/covid-1...

Yes, that's why they no longer in the control group. Informed consent can be withdrawn at any time during a trial, and people are not obligated to not take a vaccine when in the control group. The only thing you can do is remove someone from the study, or possibly move them to the other study arm.