SARS-CoV-2, the virus that causes the COVID-19 pandemic, is a major global public health challenge with severe outcomes worldwide. A robust immune system plays a crucial role in limiting the spread of the virus and supporting recovery. While several nutrients are important for immune function, vitamin D is particularly important in bolstering immunity. This review aimed to investigate the interplay between viral infection, immune response, and vitamin D in determining the prognosis of COVID-19. It is believed that vitamin D possesses anti-inflammatory, immunomodulatory, antifibrotic, and antioxidant properties, potentially aiding in the fight against SARS-CoV-2 infection. Severe complications associated with SARS-CoV-2, including cytokine storms, acute respiratory distress syndrome, and disruption of the renin-angiotensin system, are influenced by various mechanisms. Vitamin D helps regulate these processes by inhibiting inflammatory cytokines, thereby reducing disease severity and risk of mortality. Vitamin D is recommended for both prevention and support of COVID-19 treatment, because it reduces the likelihood of viral infections such as the common cold, enhances cellular immunity, modulates adaptive immunity, and increases the expression of antioxidant-related genes. However, current evidence on the direct relationship between vitamin D levels, COVID-19 severity, and mortality remains insufficient. Therefore, further randomized controlled trials and large-scale cohort studies are needed to investigate this association.
