Surely you are aware that a major part of the Clinical Trial protocol is to monitor the two groups (experimental and control) and observe the number of patients who contract the disease naturally, so that the two groups are compared. They don’t give anyone the virus, but they allow it to occur in normal course. That is how it is tested. When a sufficient number of the placebo group (control) show infection, the two populations are compared; that is how the effectiveness of 95% is determined - when the vaccine group has only 5% as much infection as the placebo group.
That is the normal procedure for clinical trials. The effectiveness IS tested.