Efficacy (the main thrust of Phase 3 trials) could be sped up and made more thorough by deliberately exposing participants. I think one could argue that deliber

Efficacy (the main thrust of Phase 3 trials) could be sped up and made more thorough by deliberately exposing participants. I think one could argue that deliberate exposure and waiting around for enough people to be "naturally" exposed are pretty equivalent in terms of end results, but that's not a winning argument so far. Might be harder to recruit when you know you're definitely getting exposed vs. no change in risk from the no-action option. But that does presume your methods of exposure are equivalent to what's happening in the real world, both in terms of method and dose. And those are both open questions here and could have bearing on efficacy.