I understand how it could be unethical, though I'd be more confident in a study that had purposeful exposure - how can we say the vaccine really was the means of protection, and not just the current measures?
If the control group intentionally avoided exposure more than the experimental group, then the vaccine is even more effective than the study result. This is a blinded study though, so participants don't know which group they're in unless they go out of their way to find out (eg. taking an antibody test).