ABSTRACT
ObjectivesSARS-CoV-2 infection induces the formation of different antibodies. However, not all of which might prevent the virus from entering the cell, although their concentrations correlate with the titers of viral neutralization tests (NTs). Antibodies against the viral nucleocapsid (NC), e.g., can be classified as such. We aimed to prove the hypothesis that the apparent correlation between NC-antibody levels and NT-titers is mediated by simultaneously occurring antibodies against viral spike-protein components. MethodsWe included 64 individuals with previous SARS-CoV-2 infection (>14d after symptom onset). SARS-CoV-2 antibodies against the NC (Roche total antibody ECLIA, Abbott IgG CMIA) and spike-protein (Technozym RBD ELISA, DiaSorin S1/S2 CLIA) were measured, and neutralization tests were performed. The effect of spike-protein antibodies on the correlation between NC-antibodies and NT-titers was evaluated by partial correlation and mediation analyses. ResultsBoth tested assays assessing antibodies against the NC correlated significantly with NT titers: Abbott {rho}=0.742, P<0.0001; Roche {rho}=0.365, P<0.01. However, when controlling the rank correlations for the presence of RBD or S1/S2 antibodies, correlation coefficients dropped to {rho}=0.318/{rho}=0.329 (P<0.05/P<0.01), respectively for Abbott and vanished for Roche. As a result, only a maximum of 11% of NT titer variability could be explained by NC-antibody levels. ConclusionsOur data suggest that the apparent correlation between NC antibodies and NT titers is strongly mediated by co-occurring RBD antibody concentrations. To avoid falsely implied causal relationships, all correlation analyses of non-spike-associated antibody assays and neutralization assays should include a partial correlation analysis to exclude a possible mediator effect of spike-associated antibodies.
Subject(s)
COVID-19ABSTRACT
Background Austria, and particularly its westernmost federal state Vorarlberg, developed the highest COVID-19 incidence rate worldwide in November 2020. Health care workers (HCW) may be at higher risk of contracting the disease within the working environment and therefore the seroprevalence in this population is of particular interest. Here, we analyzed SARS-CoV-2-specific antibody response in Vorarlberg HCW in a prospective cohort study. Methods A total of 395 HCW have been tested at three different time points for the prevalence of anti-SARS-CoV-2 IgG antibodies specific for NP and RBD. Enrollment started in June 2020 (t1), two months after the end of the first wave. Re-testing took place between October to November at the beginning of the second wave (t2), and again at the end of the second wave in January 2021(t3). Results At t1, 3% of HCW showed a strong IgG-specific responses to either NP or RBD. At t2, the rate increased to 4%, and after the second wave in January 2021, 14% had a strong response, which was assessed to be stable for up to ten months. The amount of HCW with anti-SARS-CoV-2 IgG antibodies was 38% higher than the number of infections found by PCR. Conclusion We found low numbers of SARS-CoV-2-seropositive HCW in a hotspot setting after the first wave but a massive increase during the second wave distinctly surpassing the number of infected HCW identified by PCR. Our findings, therefore, offer support for the routine application of serological testing in management of the ongoing COVID-19 pandemic.
Subject(s)
COVID-19ABSTRACT
Antibody tests are essential tools to investigate humoral immunity following SARS-CoV-2 infection. While first-generation antibody tests have primarily provided qualitative results with low specificity, accurate seroprevalence studies and tracking of antibody levels over time require highly specific, sensitive and quantitative test setups. Here, we describe two quantitative ELISA antibody tests based on the SARS-CoV-2 spike receptor-binding domain and the nucleocapsid protein. Comparative expression in bacterial, insect, mammalian and plant-based platforms enabled the identification of new antigen designs with superior quality and high suitability as diagnostic reagents. Both tests scored excellently in clinical validations with multi-centric specificity and sensitivity cohorts and showed unprecedented correlation with SARS-CoV-2 neutralization titers. Orthogonal testing increased assay specificity to 99.8%, thereby enabling robust serodiagnosis in low-prevalence settings. The inclusion of a calibrator permits accurate quantitative monitoring of antibody concentrations in samples collected at different time points during the acute and convalescent phase of COVID-19.