That’s part of it, but not the whole reason
The contribution of complement-mediated endothelial injury has been suggested, and an in vitro study found that SARS-CoV-2 spike protein could activate the alternative complement pathway [
4,5]. Studies have shown increased markers of complement activation such as C5b-9 in the circulation of individuals hospitalized with COVID-19 than in controls (healthy people, people in the intensive care unit with non-COVID-19 respiratory failure, or people with influenza) [
6-8]. Complement levels were higher in people with severe compared with moderate disease and in individuals who required mechanical ventilation compared with those who did not. Signs of complement activation in tissue biopsies have also been reported [
9,10]. The role of anti-complement therapy remains investigational.
the rest is shown on the link to UpToDate that I sent to Kevin.