Immunomodulatory Effects of Vitamin D Receptors and Serum Vitamin D Levels in COVID-19 Patients After Immunization: A Narrative Review

Coronavirus disease 2019 (COVID-19) has caused global morbidity and mortality. Vaccination remains the most effective preventive strategy. However, variation in immune response after immunization has been observed among individuals. vitamin D plays a crucial role in immune regulation through activation of the VDR, which is expressed on various immune cells. The role of vitamin D status in post-vaccination immune responses among COVID-19 patients has gained increasing scientific interest. This narrative review evaluates the immunomodulatory roles of vitamin D receptors (VDRs) and serum vitamin D status in COVID-19 patients following vaccination, with particular emphasis on immune mechanisms, inflammatory regulation, and vaccine-induced immune responses. The relevant literature concerning vitamin D, VDR signaling, COVID-19 infection, vaccination, and immune responses was comprehensively reviewed. Emphasis was placed on studies discussing immune modulation, cytokine regulation, and antibody responses. Current evidence suggests that adequate serum vitamin D status may support both innate and adaptive immune responses following COVID-19 vaccination. Activation of the VDR regulates T-cell differentiation, modulates pro-inflammatory cytokine production, and supports humoral immune responses. Vitamin D deficiency has been associated with impaired vaccine responsiveness and persistent systemic inflammation. Vitamin D and VDR-mediated signaling pathways appear to play important roles in regulating immune responses following COVID-19 vaccination. Maintaining optimal vitamin D levels may help improve vaccine-induced immunity and reduce excessive inflammatory responses.