IDO1 and COVID-19: In a recent study of COVID-19, some authors [91] reported increased activity of the tryptophan metabolic pathway, as evidenced by decreased TRP, increased KYN levels, and an increased KYN/TRP ratio [98,99], reflecting the activity of IDO [100]: acute inflammation rapidly triggers an “inflammatory storm” maintained mainly by the secretion of inflammatory cytokines, of which IL -6 is the most potent [101], enhancing the initial proinflammatory cytokine phase and suppressing the endogenous antiviral response.