Anyone else notice that Calcifediol is the most expensive vitamin-D analogue on the market? What a huge surprise that the study chose that form of vitamin-D specifically.
D3 takes 7 days to be fully metabolised to calcifediol. When Drs measure your D levels, they're actually measuring calcifediol.
I assume that the researchers picked it (1) to increase the chances of seeing a significant effect, if there is one, and (2) because if hours are at stake in saving a person's life, the fastest boost may be needed.