The ongoing Coronavirus disease 19 (Covid-19) pandemic and associated mortality in children led to an effort to address risk factors and develop protective measures. Observational studies in adults showed that vitamin D deficiency is associated with Covid-19 severity. The aim of this review was to summarise data regarding the role of serum vitamin 25(OH)D concentration in the severity of Covid-19 and the associated multisystem inflammatory syndrome in children (MIS-C). Many studies noted lower concentrations of vitamin 25(OH)D in children with Covid-19 compared with healthy controls; however, studies that assessed vitamin 25(OH)D suboptimal concentrations as a risk factor for Covid-19 severity were scarce. There was no high-quality evidence that vitamin 25(OH)D concentrations are associated with Covid-19 severity. Similarly, for MIS-C, a few studies with a small number of patients found that vitamin D deficiency was associated with more severe MIS-C. Vitamin D has many immunomodulatory actions and is consumed in the immunomodulatory cells, especially in infections such as the Covid-19 which is associated with increased inflammation and cytokine storm. Therefore, decreased concentrations of plasma vitamin 25(OH)D have been proposed to be the result of vitamin use by immunomodulatory cells in severe Covid-19, rather than a predisposing factor. In conclusion, the available data cannot prove that vitamin D deficiency is a risk factor for severe Covid-19 disease. More studies, of prospective design, are needed to investigate the role of this marker independently of other risk factors.
Keywords: MIS-C; Paediatric; Risk factor; SARS-CoV-2; Severity; Vitamin D.