I don't know if you can read the article apart from the abstract, so just in case:
It proposes the idea for the mechanism of vitamin D supplementation to better certain conditions in the short term, but worsen health long term, by acting as a immune suppressant similar to other steroids.
Here are some excerpts:
> "Consequently, if patients with autoimmune disease succeed in killing bacteria associated with their disease state, their symptoms should be expected to escalate, at least in the short-term, as cytokines and endotoxins are generated[28]. Conversely, in cases in which the immune response has been suppressed by supplementation with an immunosuppressant such as the secosteroid 25-D, one would expect to see fewer clinical manifestations of disease in the short-term, yet more advanced disease in the long-term. At a certain point, depending on the clinical symptom or physiological markers of disease, patients supplementing with vitamin D would be expected to approach a "crossover point" when additional reduction of the immune response is eclipsed by the advancing disease (Figure 2). This outcome has been demonstrated in longitudinal studies, with studies on sicker or older patients taking less time to realize the effect."
> "The Iowa Women's Health study showed vitamin D intake seemed to protect against breast cancer in the first five years after it was taken. However, the effect began to reverse between years five and ten and was completely lost after year ten, trending towards an opposing effect[29]. Lappe et al published work, conducted over four years, that seemingly showed vitamin D might lower the incidence of colorectal cancer[30]. In a similar study looking at a larger cohort and over a longer period of time, Rossouw et al found no such effect[31]."
Why can't we have nice things, eh?!
Since, vitamin D is frequently advertised as the cure all supplement, I also want to mention "antagonistic pleiotropy" as something to keep in mind, as conceptual "immunization":
https://en.wikipedia.org/wiki/Antagonistic_pleiotropy_hypoth...
Edit: OMG. I just realized, it may very well be possible that winter's VD deficiency may be needed for good health to some extent, if the body is indeed more active against lingering pathogens. This may also explain the COVID19 and VD connection, as steroids have been shown beneficial as well, maybe it's the immune suppressant quality of VD, which turns out beneficial!