I and mine relied on D3 at 900 IU per pound of body weight and all of us shed the malady quickly. We had not yet encountered the information about Zinc. We now all take the D3 and zinc daily and the other “normal” flus that have been going around here have passed us by.
My doctor prefers I not get my Vitamin D from supplements, but from the sun, or barring that, a sunlamp.