Fig. 4.

Total IgG and IgA binding (OD₄₅₀) to the SARS-CoV-2 spike protein from 48 mild COVID-19 subject's plasma. A) Total binding of IgG (left panel; circles) and IgA (right panel; squares) at baseline (BL; blue) and 6-months (6M; red) for all subjects (n=48; 1/5 plasma dilution) compared to healthy controls (n=41; grey; 1/5 plasma dilution) and a SARS-CoV-2 spike-specific antibody (purple). The limit of detection is set to the mean +3SD of the healthy controls (IgG=0·3, IgA=0·32, dotted line). The overall binding of BL, 6M and SARS-CoV-2 antibody was significantly higher than healthy controls (****p<0·0001, unpaired parametric T tests). The mean and standard deviation are shown. The dashed line is plotted at 0. B) Correlation of 50% inhibitory dilution neutralising titres (ID₅₀) against infectious culture derived SARS-CoV-2 to the spike-specific binding for IgG (left; circles; r²=0·54, p<0·0001; simple linear regression analysis) and IgA (right; squares; r²=0·10, p=0·0015; simple linear regression analysis). The dotted line on the X-axis represents the limit of detection for assays. C) Comparison of total IgG (circles) and IgA (squares) binding for the same time point in all subjects (n=96). The limit of detection (dotted line) has been set to the higher value of the two antibody isotypes (OD₄₅₀ =0·32). No one subject is observed to have both undetectable IgG and IgA at any timepoint. No significant difference was observed between the overall binding of these isotypes (p=0·090, paired-parametric T test).