I wouldn’t say quack! Id say someone who is looking for an easy answer or contribution to the problem we have at hand!
I definitely agree with you there is a ton of correlational data that provides a signal that their could absolutely be something there (for covid, MS, cancer, CV disease, and probably others). I dont however agree that there is an inkling of data that indicates any true causation. As you’re aware, thats nearly impossible to prove in the case of covid as itd be impossible to get through an IRB with the ethical dilemma of purposely exposing a group of people to covid to see if the group of people with a D level of 45-60 nmol/L was more resistant to getting infected, showing symptoms, being hospitalized, or dying.
As you and others have stated in this thread, supplementing vitamin D is such an easy, generally low risk thing to do - i do think it should be widely recommended if one could afford it (i know its cheap, but some still likely cant).
I also tend to disagree that docs are resistant to acknowledging a vitamin could cure a disease. I think theres a certain level of skepticism about huge claims made about a vitamin, but i think if theres strong data supporting anything, on average, docs will use it in their arsenal. As you mentioned - Vitamin K is used prophylactically to prevent ICH in newborns and also to reverse some blood thinners, thiamine deficiency is widely acknowledged to be causative of warnecke-korsakoff’s which has lead to it being included in the standard of care for alcoholics when they come to the ED with altered mentation, pyridoxine deficiency is accepted as a cause of peripheral neuropathy, folate and cyanocobalamine deficiencies in megaloblastic anemias. The treatment of all of those things includes the vitamin.
Perhaps claims for vitamins do have a bit of a bigger uphill battle to gain any credence in western medicine because of the outlandish claims made by such a wide variety of herbal supplements and “complimentary medicines”. A small fraction of these things actually come close to the claims made about them, so just as there is a lack of trust towards western medicine with people who are bias towards these practices, there is also a lack of trust from western medicine in the other direction.
Anyway, kind of got to rambling there, bottom line is I agree - i think theres something there for vitamin D, but still think its too soon to say theres causation.