The initial trials just need to establish that the vaccine is safe (no severe side effects) which could include those on PrEP and even those with no risk factor

The initial trials just need to establish that the vaccine is safe (no severe side effects) which could include those on PrEP and even those with no risk factor. The larger, latter trial that needs to study efficacy could simply use a large population. While HIV transmission is much more rare outside of certain high risk groups, infection does occur through accidents or deviations in behavior (e.g an EMT gets a needle prick while trying to administer Narcan). It would take a larger population and longer study but it should still be able to reach statistical significance.