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.
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).