FDA is doing exactly what’s in their statutory responsibility and authority. ie: what congress told them to do. (See my edited addendum to the parent comment.)

When someone observes that a government agency which seems to be doing something nonsensical is actually just following the law perfectly, it always brings to mind H. L. Mencken: "Democracy is the theory that the common people know what they want, and deserve to get it good and hard."

1) The FDA does not run clinical trials.

2) Trials must be large to see results because Covid is rare. The Moderna trial took 30,000 patients 3 months to get to 90 covid cases.

3) This could be a an interesting opportunity for challenge trials, but unfortunately, the key metric for vaccines is reduction in severe cases. Subjecting people likely to have a severe case is unethical when there chance of normally catching it is very low.

Why not look for key indicators of immunity in the authorized dosage group, then compare against indicators in the reduced dosage group?

This is part of my complaint. Why is a large trial, or even a challenge trial (which I also support), the only way to get results here?

There is more to science than reading statistics from trials. They should be able to ask and answer very specific questions based on the working model proven far. Then scale those results to the broader problem.

Running trials takes time. It isn't worth it.