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Are the trial participants exposed to the virus purposefully or is the result that none of participants developed covid just from regular daily living?

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There's a (blinded) control group so purely based on how deeply unethical it would be to purposefully expose them to the virus I'd wager they were 'natural' occurrences.

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?

That's precisely why there is a control group.

And that's what I mean, the control group may be avoiding exposure altogether.

The study compares how many people in the experimental group and control group develop symptomatic COVID-19. In this case 11 people in the experimental group developed COVID-19 versus 185 in the control group. Thus the vaccine is approximately 185 / (185 + 11) = 94% effective, assuming both groups behaved similarly.

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).

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