I think that the suggestion of a challenge-based Phase 3 trial is poorly elucidated, not for the reasons mentioned in other threads (the risk of severe reaction

I think that the suggestion of a challenge-based Phase 3 trial is poorly elucidated, not for the reasons mentioned in other threads (the risk of severe reactions) but because they probably would have revealed in more detail what the Phase 2/3 trials actually did reveal, that the vaccine is not 100% effective in preventing infection. (Full disclosure - I participated in the Moderna Phase 3 trial and will get vaccinated ASAP if it turns out I was in the placebo group.) The incentives for Moderna/Pfizer/AZ are a bit complex here. If they did a challenge study and discovered that 25% of vaccinated individuals were infected but had lower viral loads, that might have made it more difficult to get approved. Instead, doing what they are doing - rolling out vaccination with zero public health monitoring of folks after the fact allows them to avoid having to deal with that. Moreover, for the anti-vaxxers, who currently seem to have focused in on the fact that a tiny fraction of folks appear to have an anaphylactic response to the lipid nanoshell on the mRNA, it would have been easy to say "Oh, the vaccine doesn't actually work at all." It's unfortunate, but certainly not the first time where incentives are aligned to favor ignorance rather than maximum knowledge. I think the really interesting question I have is whether Moderna will continue to follow their Phase 3 participants after they are all unblinded and vaccinated. If they do, and do a good job of assessing changes in antibodies, then I'll be really impressed with their ethics.