Severe COVID-19 is caused by the body’s overreacting immune response and not the activity of the virus itself. Specifically, COVID-19 patients can develop Acute Respiratory Distress Syndrome (ARDS), which is respiratory failure characterized by rapid onset of a severe immune response in the lungs, and is the primary cause of death from COVID-19. In addition, COVID-19 has been known to cause a series of unusual conditions, such as stroke, heart attack, kidney failure, and multisystem inflammatory syndrome in children.
Neutrophil Extracellular Traps (NETs) are a key mediator of ARDS and have been observed in the lungs of COVID-19 patients with ARDS. Once released in the pulmonary system, NETs can cause organ damage by physical occlusion of the alveolar architecture and function as well as by activating an immune response that recruits a storm of other inflammatory cells and antibodies. In addition, NETs have been associated with the formation of blood clots (thrombosis), kidney damage and systemic inflammation among other sequela. See references below.