IDO1 and COVID-19: IDO-1 activity (as reflected by increased Kyn/Trp ratio), was markedly elevated during acute infection, which is well in line with earlier studies in patients with COVID-19,29 but also other infectious diseases (HIV, Influenza, Dengue virus, EBV)21,54, -56 and cancer.23 Increased Trp catabolism went along with enhanced neopterin formation and Phe accumulation (as reflected by Phe/Tyr), which fits well with past findings in diabetes, Dengue virus, and cancer.23,54,57