Clinical aspects of SARS-CoV-2 infection and vitamin D: COVID-19 and the endocrine system: special issue for reviews in endocrine and metabolic disorders (Felipe Casaneuva, Editor in Chief) A. Giustina and JP Bilezikian, Guest Editors

In December 2019, the first cases of severe acute respiratory syndrome due to a new coronavirus (SARS-Cov-2), later designated as Covid-19, were described in China. With rapid advance of the infection to several continents, in March 2020, WHO declared this to be a pandemic. In April 2020, the first papers suggesting a possible role of Vitamin D deficiency in the severity of this infection began to appear and dozens of articles evaluating a potential relationship of vitamin D with COVID have emerged subsequntly. This possibility was raised based on pre-existing evidence of the effects of Vitamin D on the immune system, and more specifically on acute respiratory viral infections. In addition, most Covid-19 victims belong to groups at risk for vitamin D deficiency such as the elderly, obese, chronically ill, and specific ethnic groups. Although with some contradictory reports exist, most observational and cohort studies find a relationship of low vitamin D status with greater Covid severity, others, including the few interventional studies available show inconsistent results. This paper aims to present the rapidly expanding literature to date regarding the clinical relevance of vitamin D in Covid-19 and, consequently, the reasonableness of avoiding its deficiency to keep the immune system able to respond in the best way to this acute viral infection. In the meantime, we wait for publication of several prospective randomized controlled studies that are underway, evaluating the effects of treatment with vitamin D or metabolites on the severity of Covid-19 outcomes.