Indeed, increased activity of the interferon-induced indoleamine-2,3-dioxygenase (IDO) pathway in COVID-19 patients has been well described, and this pathway is likely responsible for significant depletion of tryptophan in patients with COVID-19.38 Reduced absorption of dietary tryptophan via the small intestine in murine models of viral infection was recently shown to contribute to decreased serum tryptophan levels.39 We found a high relative abundance of genes encoding for tryptophan biosynthesis (relative to tryptophan metabolizing genes) within the human gut microbiota. This evidence concerns the gene IDO2 and viral infectious disease.