>In other words, in an observational study design, it cannot be excluded that VitD deficiency represents a surrogate marker for a general micronutrient deficien

>In other words, in an observational study design, it cannot be excluded that VitD deficiency represents a surrogate marker for a general micronutrient deficiency, which in turn reflects only the patient’s overall health status. From the authors of one of the cited papers. I.e. if you are in generally poor health already, COVID-19 is more likely to kill you.