Highlights Vitamin D may be a central biological determinant of COVID-19 outcomes. Bolus vitamin D3 supplementation during or just before COVID-19 was associated with less severe COVID-19 in frail elderly. Bolus vitamin D3 supplementation during or just before COVID-19 was associated with better survival rate in frail elderly. Randomized controlled trials are expected to firmly conclude the effect of vitamin D supplementation on COVID-19 prognosis.