I think the answer lies in the first body paragraph and the summary: Vitamin D is not recommended for preventing Covid-19; because it does not do that. Nor does it prevent Influenza and the common Cold; cousins of CoVid.
Nevertheless it does help the body withstand such virii; and the paper explains this point.
I suppose NICE and all the health services wanted to prevent false hopes that stuffing yourself with the vitamin would keep you completely safe from infection, when what it really does is improve your chances of infection being minor and recovery being full.
As we've known for years with illnesses like the cold.
