Figure 1

Serum stored on filter paper retains neutralising capacity against SARS-CoV-2 spike. Pseudo-virus particles (PVP) expressing SARS-CoV-2 spike were used to measure the neutralisation capacity of paired human sera stored in direct aliquots and eluted from dried serum spots (DSS) stored on filter paper. Neutralisation activity was defined as the serum dilution that reduced PVP infectivity by 50%, 70% or 90% (IC50, IC70 or IC90, respectively). (a–c) Slope charts from left to right display on the y-axis PVP neutralisation as (a) IC50, (b) IC70 and (c) IC90. The x-axes show the sample type with sera represented by orange circles and DSS eluates represented by blue circles. IC50, IC70 and IC90 values for paired sample types are connected via grey lines. The dotted lines across the charts represent the lower limits of detection, with the limit for direct sera in orange and DSS in blue (d–f) Scatter plots from left to right show direct sera IC50, IC70 and IC90 plotted against DSS eluates IC50, IC70 and IC90 values. Simple linear regression analysis were performed and found significant positive relationships between sera aliquots and DSS eluates IC50 values [n = 43, R² = 0.7432, p < 0.0001], IC70 values [n = 47, R² = 0.8381, p < 0.0001] and IC90 values [n = 42, R² = 0.8286, p < 0.0001]. The dotted lines represent the upper and lower 95% confidence intervals (CI) of the line of best fit. (g–i) Bland–Altman plots display on the x-axes the average IC50 values (n = 43), IC70 values (n = 47) and IC90 values (n = 42) for sera aliquots and DSS eluates and the difference between the two values on the y-axes. Red lines represent bias, and the dotted lines represent the upper and lower 95% CI.