Fig. 2.

Histological characteristics of COVID-19-associated DAD in comparison with DAD of other etiologies. a–c Key histological characteristics of COVID-19-associated DAD. a Exudative and proliferative DAD with hyaline membranes and reactive pneumocyte formation in a patient with lethal COVID-19. b Prominent capillary stasis, a hallmark of COVID-19-associated DAD. c Upon higher magnification, thrombotic occlusion can be observed in alveolar capillaries. inset Immunohistochemical staining for fibrin revealed the occlusion of alveolar capillaries by microthrombi. d An exemplary representation of DAD in a COVID-19-negative patient, notably presenting with less capillary stasis and a lack of microthrombosis. The scale bar applies to a–d: 100 µm (a, b, d), 50 µm (c).