There's a recent quasi-exprimental study on that, showing that vitamin-d is likely one of the central factors in better outcomes for severe cases of covid-19:
According to this study, vitamin-d gives better outcome than the other treatments, including hospitalization:
"Regarding care dedicated to COVID-19, only the proportion of patients who received a bolus of vitamin D3 during or just before COVID-19 differed between deceased participants and survivors, with a higher prevalence in survivors (respectively 92.2 % versus 66.7 %, P = 0.023). In contrast, there was no between-group difference in the proportion of patients treated with corticosteroids, hydroxychloroquine or dedicated antibiotics, or hospitalized for COVID-19."
And this has to do with Vitamin-D's role with the ACE2 receptor. SARS-CoV-2 has a binding affinity to ACE2, aggressively invading cells with proportionally higher ACE2 receptors (including the lungs).
In fact, someone had tried injecting hrsACE2 into someone as a treatment -- that is, letting the virus bind to hrsACE2 instead of the ACE2 receptors in the cells.
A treatment with hrsACE2 isn't generally-available, and needs a lot more study. The logsitics in producing them at scale would need to be solved, if this is a viable treatment. But this looks promising to me.
