Fig 2: Comparison of total IgG and IgG1 responses and comparison of the SFB assay and other serological assays(A–F) The total IgG response was compared with the IgG1 responses for all 109 samples from the NPHL: (A) PCR-positive and symptomatic infections (PCR-positive, symptomatic; n = 16), (B) PCR-positive and no-symptom infections (PCR-positive, none; n = 34), (C) PCR-positive and unknown symptom status infections (PCR-positive, unknown; n = 11), (D) PCR-negative and unknown symptom status infections (PCR-negative, unknown; n = 13), (E) PCR-negative/PCR not done and no-symptom infections (PCR-negative/ND, none; n = 20), and (F) PCR unknown and unknown symptom status infections (PCR unknown, unknown; n = 15). Statistical analysis was carried out using Wilcoxon matched-pairs signed-rank test. The p values for comparisons between the total IgG and IgG1 responses are shown; *p = 0.05, **p = 0.01, ***p = 0.001, ****p = 0.0001. Data are shown as mean ± SD of two independent experiments, with dotted lines indicating mean + 3 SDs of healthy donors.(G) Proportion of samples that were positive by serological tests: SFB assay, cPass, and RDT. An isotype response was defined as positive by SFB assay when the binding was more than mean + 3 SDs of the healthy controls. Samples that had a positive response for any one of the seven isotypes (IgM, IgA, IgG, IgG1, IgG2, IgG3, and/or IgG4) were defined as positive by the SFB assay.Data are shown as mean SD of two independent experiments, with dotted lines indicating mean + 3 SDs of healthy donors. +, positive; -, negative; symp, symptomatic; none, no symptoms.

Fig 3: Specific IgG subclasses against full-length S protein(A–D) Plasma samples collected from 81 individuals with COVID-19 at time points with a median of 5 days pio (n = 34), 10 days pio (n = 59), and 23 days pio (n = 66) were further screened for IgG subclasses, (A) IgG1, (B) IgG2, (C) IgG3, and (D) IgG4.(E–G) The four IgG subclass responses at a median of (E) 5 days pio, (F) 10 days pio, and (G) 23 days pio were plotted.(H–J) The proportions of individuals having a positive response at time points of (H) a median of 5 days, (I) a median of 10 days, and (J) a median of 23 days pio were analyzed.Data are shown as mean ± SD of two independent experiments, with dotted lines indicating mean + 3 SDs of healthy donors. An isotype response was defined as positive by SFB assay when the binding was more than mean + 3 SDs of the healthy controls. Statistical analysis was carried out using Kruskal-Wallis tests followed by post hoc Dunn’s multiple comparisons tests. For (A)–(D), only p values for comparisons between the different time points are shown; ∗p ≤ 0.05, ∗∗p ≤ 0.01, ∗∗∗p ≤ 0.001, ∗∗∗∗p ≤ 0.0001.