Anti-RBD immunoglobulin levels and their predictive value for SARS-CoV-2 neutralization

A large number of studies have demonstrated that anti-receptor-binding domain (RBD)-binding antibody titers correlate with SARS-CoV-2 neutralization and protection from the disease. Unlike live virus neutralization assays, antibody-binding assays are easier to perform, require lower biosafety levels, and have therefore served as a substitute for virus neutralization assays throughout the SARS-CoV-2 pandemic. Although anti-RBD antibodies are usually neutralizing, there is evidence that they can also be non-neutralizing. Moreover, different immunization regimens can vary in the induction of SARS-CoV-2-neutralizing antibodies. In this study, we hypothesized that sera from individuals with different immunization and infection histories, but with the same amount of anti-RBD total immunoglobulin, differ in their neutralizing potency. A total of 27 sera from SARS-CoV-2 convalescent, vaccinated individuals, and 27 vaccinated-only individuals were investigated by using a widely used antibody-binding assay (Elecsys anti-SARS-CoV-2 S enzyme-linked immunosorbent assay [ELISA]) and a live virus neutralization assay. As expected, anti-RBD immunoglobulin units correlated with virus neutralization capacity within the vaccine and hybrid immunized group. However, sera from both groups with matched anti-RBD units varied significantly in their neutralization potential. In detail, our data indicate a significantly higher neutralization potency of hybrid immunity compared to vaccinated-only sera with similar anti-RBD immunoglobulin levels. Our study highlights the need for cautious interpretation of quantitative antibody data from anti-RBD ELISAs, especially when comparing differently immunized groups. In other words, very similar anti-RBD levels can show very different functional activity. This finding has implications for determining possible future correlates of protection.

Throughout the SARS-CoV-2 pandemic, neutralizing antibody levels have been central to predict a protective immune response. Anti-receptor-binding domain (RBD) enzyme-linked immunosorbent assays (ELISAs) correlate with neutralization assays and are due to the integration of simple performance with timely results used as surrogate assays. However, previous studies determining correlation used homogeneous cohorts. We reevaluated the correlation of a frequently used anti-RBD ELISA and a live virus neutralization assay using a heterogeneous cohort consisting of a vaccinated group without prior SARS-CoV-2 infection and a vaccinated convalescent group. The neutralizing capacity of sera with matched anti-RBD units significantly differed between groups, decreasing the correlation of the assays. Our findings highlight the necessity of considering the immunization context when interpreting serological tests and suggest that different immunization groups may require distinct protective thresholds. Considering the immunization history, we can develop more accurate predictions of immunity not only for SARS-CoV-2 but also for future challenges.