Fig. 2

IgG ELISA OD ratio and log (IgG titer) by IIP among COVID-19 patients and controls. The scatter plot displays optical density ratios by enzyme-linked immunosorbent assay for IgG and the natural logarithm of IgG titers by indirect immunoperoxidase assay among COVID-19 patients and controls. A fitted curve prediction model with fractional polynomials and a 95% confidence interval range are also depicted. The estimated Spearman’s rank correlation coefficient is 0.778 (95% confidence interval: 0.714–0.830), which indicates a strong correlation