Comprehensive Comparison of Seven SARS-CoV-2-Specific Surrogate Virus Neutralization and Anti-Spike IgG Antibody Assays Using a Live-Virus Neutralization Assay as a Reference

Neutralizing antibodies (nAbs) are considered a valuable marker for measuring humoral immunity against SARS-CoV-2. However, live-virus neutralization tests (NTs) require high-biosafety-level laboratories and are time-consuming. Therefore, surrogate virus neutralization tests (sVNTs) have been widely applied, but unlike most anti-spike (S) antibody assays, NTs and sVNTs are not harmonized, requiring further evaluation and comparative analyses. This study compared seven commercial sVNTs and anti-S-antibody assays with a live-virus NT as a reference, using a panel of 720 single and longitudinal serum samples from 666 convalescent patients after SARS-CoV-2 infection. The sensitivity of these assays for detecting antibodies ranged from 48 to 94% after PCR-confirmed infection and from 56% to 100% relative to positivity in the in-house live-virus NT. Furthermore, we performed receiver operating characteristic (ROC) curve analyses to determine which immunoassays were most suitable for assessing nAb titers exceeding a specific cutoff (NT titer, ≥80) and found that the NeutraLISA and the cPass assays reached the highest area under the curve (AUC), exceeding 0.91. In addition, when the assays were compared for their correlation with nAb kinetics over time in a set of longitudinal samples, the extent of the measured decrease of nAbs after infection varied widely among the evaluated immunoassays. Finally, in vaccinated convalescent patients, high titers of nAbs exceeded the upper limit of the evaluated assays' quantification ranges. Based on data from this study, we conclude that commercial immunoassays are acceptable substitutes for live-virus NTs, particularly when additional adapted cutoffs are employed to detect nAbs beyond a specific threshold titer. IMPORTANCE While the measurement of neutralizing antibodies is considered a valuable tool in assessing protection against SARS-CoV-2, neutralization tests employ live-virus isolates and cell culture, requiring advanced laboratory biosafety levels. Including a large sample panel (over 700 samples), this study provides adapted cutoff values calculated for seven commercial immunoassays (including four surrogate neutralization assays and a protein-based microarray) that robustly correlate with specific titers of neutralizing antibodies.

Reduced Sensitivity of Commercial Spike-Specific Antibody Assays after Primary Infection with the SARS-CoV-2 Omicron Variant

The SARS-CoV-2 Omicron variant is characterized by substantial changes in the antigenic structure of the Spike (S) protein. Therefore, antibodies induced by primary Omicron infection lack neutralizing activity against earlier variants. In this study, we analyzed whether these antigenic changes impact the sensitivity of commercial anti-SARS-CoV-2 antibody assays. Sera from 37 unvaccinated, convalescent individuals after putative primary Omicron infection were tested with a panel of 20 commercial anti-SARS-CoV-2 immunoassays. As controls, we used samples from 43 individuals after primary infection with the SARS-CoV-2 ancestral wild-type strain. In addition, variant-specific live-virus neutralization assays were used as a reference for the presence of SARS-CoV-2-specific antibodies in the samples. Notably, in Omicron convalescents, there was a statistically significant reduction in the sensitivity of all antibody assays containing S or its receptor-binding-domain (RBD) as antigens. Furthermore, antibody levels quantified by these assays displayed a weaker correlation with Omicron-specific neutralizing antibody titers than with those against the wild type. In contrast, the sensitivity of nucleocapsid-protein-specific immunoassays was similar in wild-type and Omicron-infected subjects. In summary, the antigenic changes in the Omicron S lead to reduced immunoreactivity in the current commercial S- and RBD-specific antibody assays, impairing their diagnostic performance. IMPORTANCE This study demonstrates that the antigenic changes of the SARS-CoV-2 Omicron variant affect test results from commercial Spike- and RBD-specific antibody assays, significantly diminishing their sensitivities and diagnostic abilities to assess neutralizing antibodies.