evidence: A retrospective case-control study by Jude et al indicated that vitamin D deficiency can be linked to a greater likelihood of hospitalization in patients with COVID-19. Results from a primary cohort revealed that in hospitalized patients with COVID-19, the median serum level of 25(OH)D was 35.0 nmol/L, compared with 50.0 nmol/L in nonhospitalized individuals with COVID-19. The investigators suggested that the risk of hospitalization for COVID-19 could be reduced via widespread testing for and treatment of serum 25(OH)D insufficiency or deficiency. [67] A retrospective observational study by De Smet et al indicated that patients with COVID-19 who have vitamin D deficiency at hospital admission have an almost four-fold greater chance of mortality from COVID-19. This increase in the study occurred independently of age, ethnicity, the presence of chronic lung disease, or the degree of lung damage (as determined through chest computed tomography [CT] scan severity score). [68, 69] A retrospective, single-institution study by Dror et al of hospitalized patients with COVID-19 found that those who had a 25(OH)D level of less than 20 ng/mL prior to infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, had a 14-fold greater likelihood of severe or critical COVID-19 than did those who had a 25(OH)D level of 40 ng/mL or greater before infection. Moreover, the mortality rate during hospitalization in patients with pre-infection vitamin D deficiency was 25.6%, compared with 5.0% in those with 40 ng/mL or more of 25(OH)D. [70]