Vitamin D has a history of large scale studies pointing both to and away from health benefits, and many western countries already recommend supplementing Vitamin D during winter months. An extensive summary of the evidence for Vitamin D’s various claimed health effects can be found here.
Recently, some evidence has emerged which suggests that Vitamin D may have a protective effect against coronavirus. As Vitamin D supplementation is widely regarded as very safe, and Vitamin D pills are cheap, this has led many people to start taking Vitamin D as a preventative measure. A summary of the case for the protective effect of Vitamin D against Coronavirus is available here. More discussion is available here.
This question asks:
Will "best practice" in treating Coronavirus involve administering Vitamin D before the end of the pandemic?
This question resolves positively if, before the end of the coronavirus pandemic, the BMJ's treatment algorithm for COVID-19 recommends administering Vitamin D to patients with coronavirus.
The BMJ's treatment algorithm has different procedures for mild, moderate, severe, and critical. A recommendation to use Vitamin D for any one of these is sufficient for positive resolution.
"1st line", and "Plus" count as a recommendation. "Consider" does not.
The end of the coronavirus pandemic is considered to be the end of the first week in which total global daily deaths from coronavirus are below 250.
Several other questions about Vitamin D and Coronavirus have also been written, you can find links to them below.