Actually, I think the 3 week gap was initially a minimum. They didn’t want people getting the 2nd does before that. But then for some reason, it was adopted as the suggested time, probably in a hurry to get everyone vaccinated
It was an incredibly stupid decision. The first dose is 80% effective and 80% of “events” are after the 2nd dose probably because they are too damn close together.
They could have given twice as many people a first dose with almost minimal side effects without all the folderol of the rollout by eligibility cohorts.
Probably could have managed 80-90% single dose coverage before the variants started popping up. Then a 2nd dose booster 4-6 months later. We’d be done.