evidence: A systematic review by Borsche et al. used data from 8 studies to determine a potential optimal vitamin D level to minimize COVID-19 mortality. The study confirmed an inverse correlation between COVID-19 mortality and vitamin D levels, which showed that as vitamin D levels increase, COVID-19 mortality decreases, to a point where the projected mortality rate reaches zero at a vitamin D level of around 50 ng/ml (125 nmol/L). This paper reviews how [...] A study by Karanova et al. found a significant relationship between low vitamin D levels and severity and death among patients hospitalized with COVID-19. This study included 133 patients whose vitamin D levels at hospital admission ranged from 3 ng/ml to 97 ng/ml. 81% of the patients had a moderate course of disease. Of those suffering a severe course of COVID-19, 35% were severely vitamin D deficient and 15% were mildly vitamin D deficient. Only one of the 18 patients (6%) who died from COVID-19 had a vitamin D level greater than 20 ng/ml (50 nmol/L). This study concluded that disease severity and death due to COVID-19 were both associated with vitamin D deficiency (levels below 20 ng/ml or 50 nmol/L), with increased significance among those whose vitamin D level was severely deficient, below 10 ng/ml (25 nmol/L). Those with vitamin D levels less than 10 ng/ml had more than 4 times the risk of severe disease and almost 6 times the risk of dying from COVID-19 compared to those 10 ng/ml and higher.