TABLE 1

Clinical classification of COVID-19 infection, and the role of complement activation

Disease severity	Percentage of cases	Features/role of complement activation
Asymptomatic	20%–40%	Immune system shuts off the virus; no clinical signs or symptoms of infection; immunity induced; “natural” vaccination Complement activation beneficial
Upper respiratory tract (cold-like syndrome)	30%–70%	Virus replicates in upper-airway tissues leading to sore throat, cough, fever, sneezing, altered taste and smell, “typical cold” syndrome; usually lasts 2 to 5 days; recovery in 2 to 3 weeks Complement activation beneficial^{^a}
Moderately severe	5%–10%	Viral load is greater; extension to deeper parts of the lung (pneumonia) and possibly to other tissues, with prominent inflammatory picture Complement activation not beneficial
Severe	1%–5%^{^a}	Heavy viral load; extension to extrapulmonary tissues, especially endothelial cell populations; thromboembolic phenomena Virus is never adequately controlled^{^b} Virus eventually controlled and eliminated, but often with residual effects Complement activation poor for host

↵a In these patients, early in the course of the infection, immune enhancement may be indicated.
↵b In these patients, dampening the inflammatory response could be advantageous, especially if the viral replication has been controlled.