University of Chicago Medicine found an association between vitamin D deficiency and the likelihood of becoming infected with the coronavirus.

Patients whose vitamin D levels were tested last year to be deficient (< 20ng/ml and untreated were twice as likely to test positive for the COVID-19 compared to patients who had sufficient levels of the vitamin.

https://www.uchicagomedicine.org/forefront/coronavirus-disease-covid-19/vitamin-d-deficiency-may-raise-risk-of-getting-covid19