The following narrative review presents evidence of vitamin D supplementation and the potential reduction of risk for influenza and COVID-19 related illness and mortality. Vitamin D deficiency is common and has been linked to acute respiratory distress syndrome (ARDS) severity, a symptom of COVID-19, and increased mortality alongside the presence of chronic disease and advanced age. There are a variety of mechanisms which implicate vitamin D and its role in immune system activation including: lower viral replication via cathelicidin and defensin production, decreasing the cytokine/chemokine storm which interfere with the lining of the pulmonary epithelium potentially leading to pneumonia, and increasing the expression of antioxidant related genes. Authors recommend for people at an elevated risk of influenza and COVID-19 to take 10,000 IU/d of vitamin D3 for a few weeks, followed by 5000 IU/d with the intention of raising serum levels of 25-hydroxyvitamin D above 100–150 nmol/L, and reduce the risk of infection.The following narrative review presents evidence of vitamin D supplementation and the potential reduction of risk for influenza and COVID-19 related illness and mortality. Vitamin D deficiency is common and has been linked to acute respiratory distress syndrome (ARDS) severity, a symptom of COVID-19, and increased mortality alongside the presence of chronic disease and advanced age. There are a variety of mechanisms which implicate vitamin D and its role in immune system activation including: lower viral replication via cathelicidin and defensin production, decreasing the cytokine/chemokine storm which interfere with the lining of the pulmonary epithelium potentially leading to pneumonia, and increasing the expression of antioxidant related genes. Authors recommend for people at an elevated risk of influenza and COVID-19 to take 10,000 IU/d of vitamin D3 for a few weeks, followed by 5000 IU/d with the intention of raising serum levels of 25-hydroxyvitamin D above 100–150 nmol/L, and reduce the risk of infection.