Vitamin D may help boost the innate immune system in a number of ways, explained Dr. Adit Ginde, a professor of emergency medicine at the University of Colorado School of Medicine and one of the study's authors. One mechanism, he says, is by increasing antimicrobial peptides, which function as natural antibiotic and antiviral guards against pathogens. Though some researchers are not yet convinced of the evidence for vitamin D and respiratory illness, others, like Ginde, are. "Based on those mechanisms, prevention [of COVID-19] would be the first scenario that you would expect to work," says Ginde. "It's also very clear deficiency causes dysfunction in the immune system."