C5 and COVID-19: Together with an increase in the amounts of plasma C-reactive protein (CRP), IL-6, and the chemokines CCL4 (macrophage inflammatory protein-1β), CCL2 (monocyte chemoattractant protein 1) and CXCL9 (monokine induced by interferon-γ), the levels of soluble C5a were increased according to the severity of COVID-19.