S100A9 and COVID-19: To investigate the association between a variety of neutrophil activation–associated biomarkers and COVID-19 severity, we measured the concentrations of myeloperoxidase (MPO), MMP-9, S100 calcium binding protein A9 (S100A9), lipocalin-2 (also known as neutrophil gelatinase-associated lipocalin, NGAL), lactoferrin, IL-8, and IL-16 in SARS-CoV-2–infected patients and HVs (Figure 2 and Supplemental Table 3).