For the S3-ACE2 neutralization assay, a lower limit half maximal inhibitory concentration (IC50) serum dilution of 50 was set as the specificity cutoff using IC50 values for the 104 pre–COVID-19 pandemic healthy donor samples (50 cutoff = 12.5 mean IC50 + 4 × 9.0 SD to minimize detection of false-positive samples; Fig. 1B). Here, ACE2 is linked to COVID-19.