Fig 2: Analysis of spike-specific IgG and IgA reactivity in convalescent and control cohorts.

Spike-specific IgG (a–b) and IgA (d–e) responses at the indicated serum dilutions were determined for convalescent (Conv, n=197) and control (Ctrl, n=216) cohorts. (c, f) Inter-assay reproducibility of independent IgG and IgA assays at serum dilutions of 1:1,000 and 1:200, respectively, was assessed by a linear regression analysis. (g–h) Spike-specific IgG and IgA reactivity for Conv and Ctrl cohorts at the selected test dilution (1:1,000 and 1:200 serum dilutions, respectively). Diagnostic thresholds for IgG and IgA tests are shown as dotted lines (A₄₅₀ values of 0.90 and 0.60, respectively). Ctrl cohort is separated into Pre-2020 (n=171) and Jan 2020 groups (n=45). (i) IgG and IgA reactivities of each sample in the Conv (orange circles) and Ctrl (green circles). Respective diagnostic thresholds are indicated as dotted lines. Percentages reflect the proportion of Ctrl and Conv in each quadrant.