C4A and COVID-19: Median values of C3 and C4 levels in critical COVID-19 patients (group 3) were 116 mg/dL (108.0–127.7) and 26.0 mg/dL (17.0–39.0), respectively, whereas in mild and severe patients (groups 1 and 2), the median values were 125 mg/dL (105.5–145.5) and 26.5 mg/dL (21.0–33.0), respectively (p > 0.05).