IDO2 and COVID-19: According to the SHAP values, the disrupted metabolomic profile of acute COVID-19 (see Figures 4B, 5A) is primarily associated with metabolites participating in the immune response and energy metabolism based on our top metabolites found, for example, elevated SHAP values for metabolites such as Kynurenine, a by-product of the tryptophan metabolism pathway, suggest activation of indoleamine 2,3-dioxygenase (IDO) due to inflammation (Kim et al., 2015).