SARS-CoV-2 Serologic Assays Dependent on Dual-Antigen Binding Demonstrate Diverging Kinetics Relative to Other Antibody Detection Methods

Abstract

Longitudinal studies assessing durability of the anti-severe acute respiratory syndrome coronavirus 2 (anti-SARS-CoV-2) humoral immune response have generated conflicting results. This has been proposed to be due to differences in patient populations, the lack of standardized methodologies, and the use of assays that measure distinct aspects of the humoral response. SARS-CoV-2 antibodies were serially measured in sera from a cohort of 44 well-characterized convalescent plasma donors over 120 days post-COVID-19 symptom onset, utilizing eight assays, which varied according to antigen source, the detected antibody isotype, and the activity measured (i.e., binding, blocking, or neutralizing). While the majority of assays demonstrated a gradual decline in antibody titers over the course of 120 days, the two electrochemiluminescence immunoassay Roche assays (Roche Diagnostics Elecsys anti-SARS-CoV-2 [qualitative, nucleocapsid based] and Roche Diagnostics Elecsys anti-SARS-CoV-2 S [semiquantitative, spike based]), which utilize dual-antigen binding for antibody detection, demonstrated stable and/or increasing antibody titers over the study period. This study is among the first to assess longitudinal, rather than cross-sectional, SARS-CoV-2 antibody profiles among convalescent COVID-19 patients, primarily using commercially available serologic assays with Food and Drug Administration emergency use authorization. We show that SARS-CoV-2 antibody detection is dependent on the serologic method used, which has implications for future assay utilization and clinical value.

Keywords: COVID-19; SARS-CoV-2; antibodies; antibody; dual-antigen binding assays; immunoassays; serology.

FIG 1

SARS-CoV-2 antibody titers relative to time post-symptom onset. (A to H) Assay-specific antibody levels for each donor sample (individual filled circles) were plotted as a function of time post-symptom onset. Results were binned at 30 to 60 days (orange), 60 to 90 days (blue), and 91+ days (green). Data collected prior to day 30 are shown in gray due to limited sample size (n = 12). The horizontal boxplot lines indicate the median values, boxes indicate the 25th and 75th percentiles, and vertical lines indicate the largest values within 1.5 times the interquartile range (IQR) greater than the 75th percentile and the smallest values within 1.5 times the IQR lower than the 25th percentile. The horizontal dotted lines indicate the positive cutoff threshold of each assay. Mayo Clinic nAb is shown on the log₂ scale. Abbreviations: S/Co, signal to cutoff; NC, nucleocapsid protein; ECLIA, electrochemiluminescent immunoassay; CIA, chemiluminescent immunoassay; RBD, receptor binding domain of spike glycoprotein; nAb, neutralizing antibody; NC, nucleocapsid protein; S, spike glycoprotein; S1, subunit 1 of spike glycoprotein; RCU, relative concentration unit.

FIG 2

Longitudinal SARS-CoV-2 antibody levels in COVID-19 convalescent plasma donors. (A to H) Data plotted are the individual patient (n = 44) linear trajectories (in gray) for specimens collected after onset of COVID-19. Table S2 in the supplemental material presents the slope estimates and P values associated with the trends. The bold red lines represent the model-based reference lines for the fixed effects. See Fig. 1 legend for abbreviations.

FIG 3

SARS-CoV-2 antibody titer correlation across assays. The correlation of titers between different assays is shown in a matrix format using the Kendall rank correlation τ_p. Blue indicates a higher degree of correlation and red indicates a lower degree of correlation.