IDO1 and COVID-19: In summary, different morphological and immunophenotypical vascular patterns can be defined in COVID-19, where the IDO1+ and IDO1-/IDO2+ phenotypes may correspond, in our view, to the previously defined biphasic presentation of COVID-19 pneumonia, with an early type-L pattern, characterized by vascular enlargement, preserved compliance, hypoxemia, and an out-of-proportion hypocapnia; and a more severe and potentially fatal type-H pattern, characterized by reduced vascular relaxation and vascular dysfunction [31,247,248,249,250].