Exploring links between vitamin D deficiency and COVID-19

Coronavirus Disease 2019 (COVID-19) pandemic remains a major public health threat in most countries.The causative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus can lead to acute respiratory distress syndrome and result in mortality in COVID-19 patients.Vitamin D is an immunomodulator hormone with established effectiveness against various upper respiratory infections.Vitamin D can stall hyper-inflammatory responses and expedite healing process of the affected areas, primarily in the lung tissue.Thus, there are ecological and mechanistic reasons to promote exploration of vitamin D action in COVID-19 patients.As no curative drugs are available currently for COVID-19, we feel that the potential of vitamin D to alter the course of disease severity needs to be investigated.Clinical studies may be undertaken to address the value of vitamin D supplementation in deficient, high-risk COVID-19 patients. SARS-CoV-2 infection and the cytokine stormThere are two critical questions that emanate from the title of this article.The first one is whether there is an association between vitamin D deficiency and susceptibility to Coronavirus Disease 2019 (COVID-19).The second is whether vitamin D administration to deficient individuals can prevent infection or alter the course of disease severity.Here, we have collated available evidence that addresses both these aspects of vitamin D in relation to COVID-19.The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects pulmonary epithelial cells using the angiotensin converting enzyme-2 (ACE-2) receptor [1].Besides pulmonary epithelial damage, SARS-CoV-2 also infects macrophages through ACE-2 receptors and activates them [2].Macrophages, neutrophils, and T cells get activated through sustained elevation of cytokines including interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF) alpha, resulting in type 2 pneumocyte apoptosis, and in some patients a path that leads to acute respiratory distress syndrome (ARDS) [2].The host responses are sometimes amplified by an overwhelming expression of pro-inflammatory cytokines [3].This 'cytokine storm' is responsible for some of the serious manifestations of COVID-19 such as ARDS (Fig 1A) [3].Hypoxemia and bilateral lung infiltration are features reminiscent of severe viral pneumonia that result from endothelial injury, excessive cytokines, and immune overkill [3,4].