A Label-Free SARS-CoV-2 Surrogate Virus Neutralization Test and a Longitudinal Study of Antibody Characteristics in COVID-19 Patients

Luo, Y. R.; Yun, C.; Chakraborty, I.; Wu, A. H. B.; Lynch, K. L.

Background. The laboratory-based methods to measure the SARS-CoV-2 humoral response include virus neutralization tests (VNTs) to determine antibody neutralization potency. For ease of use and universal applicability, surrogate virus neutralization tests (sVNTs) based on antibody-mediated blockage of molecular interactions have been proposed. Methods. A label-free surrogate neutralization assay (LF-sVNT) was established on the TFI label-free immunoassay analyzer to measure neutralizing antibody titers in serum samples. The LF-sVNT analyzes the binding ability of RBD to ACE2 after neutralizing RBD with antibodies in serum. Results. The LF-sVNT neutralizing antibody titers (IC50) were determined from serum samples (n=246) from COVID-19 patients (n=113), as well as the IgG concentrations and the IgG avidity indices. There is considerable variability in the kinetics of the IgG concentration and neutralizing titer between individuals, however, there is an initial rise, plateau and then in some cases a gradual decline at later timepoints after 40 days post-symptom onset. The IgG avidity, in the same cases, plateaued after the initial rise and did not show a corresponding decline with IgG concentration and neutralizing ability. Conclusions. The LF-sVNT can be valuable tool in clinical laboratories for the assessment of the presence of neutralizing antibodies to COVID-19. This study is the first to provide longitudinal neutralizing antibody titers beyond 200 days post symptom onset. Despite the decline of IgG concentration and neutralizing antibody titer, IgG avidity index increases, reaches a plateau and then remains constant up to 8 months post-infection. The decline of antibody neutralization potency can be attributed to the reduction in antibody quantity rather than the deterioration of antibody avidity, a measure of antibody quality.