Here is the ultimate and comprehensive summary written by two of the best vitamin D experts – Martineau and Cantorna
… lack of a consistent protective signal from RCTs reporting so far is reflected by the absence of any recommendation relating to prophylactic or therapeutic use of vitamin D for COVID-19 in guidelines from national or international bodies … If vitamin D does have favourable immunomodulatory effects in COVID-19, then demonstrating a meaningful clinical benefit of supplementation over existing standards of care is likely to become increasingly challenging, as ever more effective pharmacological therapies and vaccines emerge.