There was substantial deposition of activated complement products (C5b-9, C4d, MASP-2) and their co-localization with COVID-19 spike glycoproteins in the microvasculature of interalveolar septa, and pulmonary microvascular injury was associated with elevated plasma C5a and endothelial C5b-9 deposition in COVID-19 patients with ARDS (57), suggesting a key role of complement-mediated catastrophic microvascular injury syndrome in the pathogenesis of severe COVID-19. Here, C5 is linked to acute respiratory distress syndrome.