DPP4 and COVID-19: Correcting vitamin D deficiency has been associated with suppressed CD26 adhesion molecules used for COVID-19 cell adhesion and invasion, as well as being linked to the ability to attenuate IFN-γ and IL-6 inflammatory responses, both of which are highly correlated with critically ill, ventilated COVID-19 patients [229] and within the elderly, obese, and immunocompromised.