See these studies that indicate vitamin D deficiency makes for worse results and recommend taking a supplement as a preventative and treatment.
“Analysis of vitamin D level among asymptomatic and critically ill COVID-19 patients and its correlation with inflammatory markers.” Scientific Reports 10, November 2020.
“Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study.” The Journal of Steroid Biochemistry and Molecular Biology, October 2020.
“SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels.” PLOS ONE, September 2020.
“Short term, high-dose vitamin D supplementation for COVID-19 disease: a randomised, placebo-controlled, study (SHADE study). BMJ Journals, November 2020.