Fig. 3: Relationships between ppNAT titer and ELISA-binding titers among all TAb-positive samples.

a The relationship between ID50 (maximal dilution ratio at half-infection inhibition) of pseudotype lentivirus-based neutralization tests (ppNAT) and different groups of signal to the cutoff ratio (S/CO) of total antibody (TAb). The horizontal gray line shows the cutoff value of ppNAT (ID50 = 20). The neutralization titer successively elevated with increasing S/CO of TAb among all samples (p < 0.001, Group 1–5 vs. 5–10: p < 0.001; Group 1–5 vs. 10–15: p < 0.001; Group 1–5 vs. >15: p < 0.001; Group 5–10 vs. 10–15: p = 0.103; Group 5–10 vs. >15: p < 0.001; Group 10–15 vs. >15: p = 0.001). Median titers of the four groups are 11.0, 66.0, 97.0, and 233.9, respectively. b The relationship between ppNAT ID50 titer and titer of IgG antibody against receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 (IgG-RBD). The correlation coefficient (r) by Spearman rank correlation analysis between IgG-RBD and ID50 was as high as 0.844 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgG-RBD assay (S/CO = 1.0). c The relationship between ppNAT ID50 titer and titer of IgG antibody against nucleoprotein of SARS-CoV-2 (IgG-N). The correlation coefficient (r) between IgG-N and ID50 was 0.725 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgG-N assay (S/CO = 1.0). d The relationship between ppNAT ID50 titer and titer of IgM antibody against SARS-CoV-2. The correlation coefficient (r) between IgM and ID50 is 0.660 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgM assay (S/CO = 1.0). p-values by Kruskal–Wallis test followed by Bonferroni’s multiple-comparison post hoc test (two-sided) are indicated in (a), and the Spearman correlation coefficient (r) and p-values by Spearman’s test are indicated in (b–d). The error band in gray shows the 95% confidence intervals of the fitted line by Spearman rank correlation analysis in (b–d).