(c) Twenty-four human sera were then two-fold diluted and incubated with the ISA D614 and mCherry D614 strains and nAb titers were recorded at 5 days post-infection. nAb titers were defined as the highest dilution that inhibited the production of distinct cpe with the ISA D614 SARS-CoV-2 or fluorescence with the fluorescent mCherry D614 SARS-CoV-2. Each black dot represents results from a given number of sera. Statistical analyses were performed using univariate linear regression. The error band (in grey) represents the 95% confidence interval of the regression line. The Pearson correlation coefficient (R2) and P value analyses are shown. 