Coronavirus disease 2019 and vitamin D.

Vitamin D deficiency is one of the most common vitamin deficiencies across different populations. It has primarily been implicated in the development of metabolic bone disease in adults and children. However, in recent years its role in immunomodulation has also emerged and has gained further importance since the occurrence of coronavirus disease 2019 (COVID-19). Here, we describe the most recent literature on vitamin D and its impact on immunomodulatory pathways. Furthermore, the current evidence on the impact of vitamin D deficiency on COVID-19 infection, severity, and prognosis is summarised. We also highlight the key research gaps in this field that need further research.