He addresses that in the video.

The calcifediol is the active form and is utilized by the body immediately. The D3 supplement a person would take has to be converted to calcifediol in the liver and takes a few days to complete.

So for prevention taking the D3 supplement is sufficient knowing that it may take a week before it starts to raise levels in the blood. For someone who is already ill or for this study that wanted to know its effects using the calcifediol the researchers knew it would be immediately available for the body to use.

Wife and I use a supplement that has both D3 and K2 together as both have a lot to do with where calcium is deposited in the body. You want calcium in the hard tissue like bones and not in the soft tissue like heart or blood vessels.

https://www.amazon.com/gp/product/B07RX4P4RL/ref=ox_sc_saved_title_1?smid=A2CXYOV1GSSB5W&psc=1

Drops

John