High abundance of fibrin deposits in the lungs of patients with severe COVID-19. (A-B) Fibrin (red) accumulation in postmortem lung tissue sections of patients with severe COVID-19 or ARDS-influenza and donors (n = 5/group). Time from death to autopsy was matched for all groups examined. Bar represents 100 µm. (B) Randomly chosen high-magnification images of the COVID-19 and ARDS-influenza clots presented in panel A. Arrows indicate fibrin deposits. All patients available are represented. Fibrin fibers were stained with an anti-fibrinogen/fibrin antibody and then developed by incubation with fast red dye. (C) Fibrin abundance in COVID-19, ARDS-influenza, and donor lungs. From each patient, 5 images were obtained in different areas of the lung, and the percentage of total area was determined in all images (n = 5 per group). *P < .05; **P < .01. (D) Fibrin fiber density in COVID-19 and ARDS-influenza lungs. From each patient, 5 images of fibrin deposits were taken, and fibril density was determined in all of them (n = 5 per group).