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

Disease severityPercentage of casesFeatures/role of complement activation
Asymptomatic20%–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 beneficiala
Moderately severe5%–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
Severe1%–5%aHeavy viral load; extension to extrapulmonary tissues, especially endothelial cell populations; thromboembolic phenomena
Virus is never adequately controlledb
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.