Antibody Avidity and Neutralizing Response against SARS-CoV-2 Omicron Variant after Infection or Vaccination

Abstract

Background: The recently emerged SARS-CoV-2 Omicron variant exhibits several mutations on the spike protein, enabling it to escape the immunity elicited by natural infection or vaccines. Avidity is the strength of binding between an antibody and its specific epitope. The SARS-CoV-2 spike protein binds to its cellular receptor with high affinity and is the primary target of neutralizing antibodies. Therefore, protective antibodies should show high avidity. This study aimed at investigating the avidity of receptor-binding domain (RBD) binding antibodies and their neutralizing activity against the Omicron variant in SARS-CoV-2 infected patients and vaccinees.

Methods: Samples were collected from 42 SARS-CoV-2 infected patients during the first pandemic wave, 50 subjects who received 2 doses of mRNA vaccine before the Omicron wave, 44 subjects who received 3 doses of mRNA vaccine, and 35 subjects who received heterologous vaccination (2 doses of adenovirus-based vaccine plus mRNA vaccine) during the Omicron wave. Samples were tested for the avidity of RBD-binding IgG and neutralizing antibodies against the wild-type SARS-CoV-2 virus and the Omicron variant.

Results: In patients, RBD-binding IgG titers against the wild-type virus increased with time, but remained low. High neutralizing titers against the wild-type virus were not matched by high avidity or neutralizing activity against the Omicron variant. Vaccinees showed higher avidity than patients. Two vaccine doses elicited the production of neutralizing antibodies, but low avidity for the wild-type virus; antibody levels against the Omicron variant were even lower. Conversely, 3 doses of vaccine elicited high avidity and high neutralizing antibodies against both the wild-type virus and the Omicron variant.

Conclusions: Repeated vaccination increases antibody avidity against the spike protein of the Omicron variant, suggesting that antibodies with high avidity and high neutralizing potential increase cross-protection against variants that carry several mutations on the RBD.

Figure 1

RBD binding IgG titers (a), RBD binding IgG antibody avidity (b), and neutralizing antibody titers (c) to SARS-CoV-2 wt virus in SARS-CoV-2 infected patients by time after hospital admission. RBD binding IgG titers which exceeded the last dilution (>51200) were plotted as 51200 titers. The antibody avidity was expressed as avidity index (AI). Tukey boxplots show outlier values (dots), medians (middle line), and third and first quartiles (boxes), while the whiskers display the minimum and maximum values. Horizontal dashed line represents the lower limit of quantification (LLOQ) of ELISA and microneutralization (MN) assay and AI range (low, intermediate, and high). Statistically significant differences were analyzed by Kruskal-Wallis and Dunn's multiple comparisons test (p < 0.05).

Figure 2

RBD binding IgG antibody avidity and neutralizing antibody titers to SARS-CoV-2 in SARS-CoV-2 infected patients and vaccinated cohorts: (a) RBD binding IgG antibody titers against wt virus; (b) RBD binding IgG antibody avidity against wt virus; (c) neutralizing antibody titers against wt virus; (d) RBD binding IgG antibody titers against Omicron variant; (e) anti-RBD IgG antibody avidity against Omicron variant; and (f) neutralizing antibody titers against Omicron variant. The antibody avidity was expressed as avidity index (AI). Tukey boxplots show outlier values (dots), medians (middle line), and third and first quartiles (boxes), while the whiskers display the minimum and maximum values. Horizontal dashed line represents the lower limit of quantification (LLOQ) of microneutralization (MN) assay and AI range (low, intermediate, and high). Statistically significant differences were analyzed by Kruskal-Wallis and Dunn's multiple comparisons test (p < 0.05).

Figure 3

Multiple regression model of antibody avidity and neutralizing antibody titres. Microneutralization (MN) titers are expressed as log2 of the normalized data. Antibody avidity results are expressed as normalized data of avidity index (AI). Regression of MN titers on AI with the virus strain as the dummy variable, Omicron variant (a), and wt virus (b) proved significant for both strains. The number of vaccine doses was included in the regression analysis as a variable: 2 vaccine doses in the top panels, 3 doses in the bottom panels of the Omicron variant (c) and wt virus (d).