There was no significant difference in classical CS activation or CS C3 levels between COVID-19 patients and the control group (CH50: 88.47 [± 22.1] % vs. 99.85 [± 22.6] %; p = 0.06; C3: 1.51 [1.35–1.83] g/L vs. 1.53 [± 0.4252] g/L; p = 0.3049). Here, C3 is linked to COVID-19.