Findings: In this double-blind, randomized, placebo-controlled trial involving 240 hospitalized patients with severe COVID-19, a single dose of 200,000 IU of vitamin D3 supplementation was safe and effective in increasing 25-hydroxyvitamin D levels, but did not significantly reduce hospital length of stay (hazard ratio, 1.12) or any other clinically-relevant outcomes compared with placebo.

Single dose is not the way other findings are using vD3. vD3 is used thought the body for many things and you need daily supplament to get and keep the concentration up over time