C3 and COVID-19: A significantly positive correlation was found between levels of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, and TNF-α) and complement factors (C3a, C5a, and factor P (properdin) in critically ill COVID-19 patients with associated CS compared to moderately ill patients, henceforth these findings reflect the potential utility of complement factors to serve as biomarkers of severity of disease progression in COVID-19 associated CS [13].