Longitudinal Serological Analysis and Neutralizing Antibody Levels in Coronavirus Disease 2019 Convalescent Patients

Abstract

Background: Understanding the longitudinal trajectory of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies is crucial for diagnosis of prior infection and predicting future immunity.

Methods: We conducted a longitudinal analysis of coronavirus disease 2019 convalescent patients, with neutralizing antibody assays and SARS-CoV-2 serological assay platforms using SARS-CoV-2 spike (S) or nucleocapsid (N) antigens.

Results: Sensitivities of serological assays in diagnosing prior SARS-CoV-2 infection changed with time. One widely used commercial platform that had an initial sensitivity of >95% declined to 71% at 81-100 days after diagnosis. The trajectories of median binding antibody titers measured over approximately 3-4 months were not dependent on the use of SARS-CoV-2 N or S proteins as antigen. The median neutralization titer decreased by approximately 45% per month. Each serological assay gave quantitative antibody titers that were correlated with SARS-CoV-2 neutralization titers, but S-based serological assay measurements better predicted neutralization potency. Correlation between S-binding and neutralization titers deteriorated with time, and decreases in neutralization titers were not predicted by changes in S-binding antibody titers.

Conclusions: Different SARS-CoV-2 serological assays are more or less well suited for surveillance versus prediction of serum neutralization potency. Extended follow-up should facilitate the establishment of appropriate serological correlates of protection against SARS-CoV-2 reinfection.

Keywords: COVID-19; Neutralizing antibodies; SARS-CoV-2; Serology.

Figure 1.

Longitudinal analysis of serum samples from participants with coronavirus disease 2019. A, Sensitivity of the Abbott, DiaSorin, Siemens, and Roche serological assays, measured in samples collected at 4 time points after polymerase chain reaction (PCR) testing, with 95% confidence intervals (CIs). B, Relative antibody titers for the DiaSorin, Siemens, Abbott, and Roche assays at visits 1–3, normalized to visit 1 (serological assay values are reported in the assay-specific units of measurement and are expressed as % of visit 1). Horizontal lines indicates median values with 95% CIs. Statistical significance was assessed with the Wilcoxon test. C, Values for DiaSorin, Siemens, Abbott, and Roche serological assays for each participant plotted over time (each line represents 1 participant) (serological assay values are reported in the assay-specific units of measure). Assay thresholds are indicated by dotted horizontal lines. Abbreviations: AU, arbitrary units; COI, cutoff index; NS, not significant; s/c, sample to calibrator index.

Figure 2.

Neutralization activity in serum samples from participants with coronavirus disease 2019. A, Half-maximal neutralization titer (NT₅₀) for each individual participant, measured in serum samples collected at 3 visits. Inset shows correlation of NT₅₀ values for samples collected at each visit, with Spearman r values (P < .001). B, Relative NT₅₀ values in serum samples obtained at visits 1–3, normalized to visit 1. Horizontal line represents median with 95% confidence interval. Statistical significance was assessed with the Wilcoxon test. C, Frequency of serum samples with NT₅₀ values falling to various quantitative categories at each visit. D, NT₅₀ values for each participant plotted over time (each line represents 1 participant). The limit of detection is indicated by a dotted horizontal line. Abbreviation: PCR, polymerase chain reaction.

Figure 3.

Correlation of serological results with neutralization titers. A–D, Serological assay values for the DiaSorin (A), Siemens (B), Abbott (C) and Roche (D) assays (serological assay values are reported in the assay-specific units of measure) versus half-maximal neutralizing titer (NT₅₀) values. Samples collected at each visit are indicated by color and are plotted individually as well as on a composite graph. Spearman r values for all visits (black) and for individual visits are indicated (P < .001). Abbreviations: AU, arbitrary units; COI, cutoff index; s/c, sample to calibrator index.

Figure 4.

Lack of correlation of changes in serological results with changes in neutralization titers. A–D, Fold change (from visit 1 to visit 3) in serological assay values (reported in the assay-specific units of measure) for the DiaSorin (A), Siemens (B), Abbott (C), and Roche (D) assay versus fold change in half-maximal neutralizing titer (NT₅₀) values. Spearman r and P values are shown. Abbreviations: AU, arbitrary units; COI, cutoff index; s/c, sample to calibrator index.