https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7965847/

Tests from multiple countries (Korea, Spain, UK, Israel) showed strong correlation between COVID-19 positive results with people who were Vitamin D deficient. Also, it appears the severity of the symptoms were also affected by this as well. There was a study in Italy which showed no correlation between COVID-19 test results or severity but this may have been due to how subjects were recruited for the study.

This indicates that there might be the legacy effect of being vitamin D-sufficient and that raising serum 25(OH)D concentrations over a short period of time might not be as effective as maintaining serum 25(OH)D concentrations in a preferred range of 40 to 60 ng/mL (100-150 nmol/L) over the long term.

Given the promising evidence on the potential benefit of vitamin D against COVID-19, a number of ongoing randomized controlled trials have been conducted with the aim to investigate the impact of vitamin D supplementation of different forms and dosing regimens. A pilot randomized clinical trial gave oral 25(OH)D3 (calcifediol) or placebo to 76 patients with COVID-19 and showed that the treatment group had a markedly reduced rate of intensive care unit admission (2% vs 50%, P < .001).108 However, in a larger randomized controlled trial giving 240 hospitalized patients with moderate-to-severe COVID-19 200 000 IUs of vitamin D3 or placebo, there were no differences in length of hospital stay, in-hospital mortality, admission to intensive care unit, or mechanical ventilation requirement.109 This emphasizes that the immunomodulatory effects of vitamin D are likely to be the results of its long term rather than short-term actions.