A longitudinal seroprevalence study in a large cohort of working adults reveals that neutralising SARS-CoV-2 RBD-specific antibodies persist for at least six months independent of the severity of symptoms
Authors: Wagner Angelika , Guzek Angela , Ruff Johanna , Jasinska Joanna , Scheikl Ute , Zwazl Ines , Kundi Michael , Stockinger Hannes , Farcet Maria R. , Kreil Thomas R. , Hoeltl Eva , Wiedermann Ursula 
Published: December 2020

AbstractBackgroundIn spring 2020, at the beginning of the first pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) wave in Europe, we set up an assay system for large-scale testing of virus-specific and protective antibodies including their longevity.MethodsWe analysed the sera of 1655 adult employees for SARS-CoV-2-specific antibodies using the S1 subunit of the spike protein of SARS-CoV-2. Sera containing S1-reactive antibodies were further evaluated for receptor-binding domain (RBD)- and nucleocapsid protein (NCP)-specific antibodies in relation to the neutralisation test (NT) results at 0, three and six months.FindingsWe found immunoglobulin G (IgG) and/or IgA antibodies reactive to the S1 protein in 10.15% (n=168) of the participants. In total, 0.97% (n=16) were positive for S1-IgG, 0.91% (n=15) were S1-IgG-borderline and 8.28% (n=137) exhibited only S1-IgA antibodies. Next, we evaluated the 168 S1-reactive sera for RBD- and NCP specificity: 8.33% (n=14) had detectable RBD-specific and 6.55% (n=11) NCP-specific antibodies. The latter correlated with NTs (kappa coefficient = 0.8660) but started to decline already after 3 months. RBD-specific antibodies correlated best with the NT (kappa = 0.9448) and only these antibodies were stable for up to six months. All participants with virus-neutralising antibodies reported symptoms, of which, anosmia and/or dysgeusia correlated best with the detection of virus-neutralising antibodies.InterpretationRBD-specific antibodies were most reliably detected post infection, independent of the number/severity of symptoms, and correlated best with protective neutralising antibodies at least for six months. They thus qualify best for large-scale seroepidemiological evaluation of both seroprevalence and seroprotection.FundingThis study received funding from the Austrian Ministry of Education, Science and Research within the research framework in relation to the coronavirus disease 2019 pandemic (GZ 2020 0225 104).Key pointsPersistence of SARS-CoV-2 antibodies depends on their specificity. Total RBD-specific antibodies are those that are stable for up to at least six months and correlate best with neutralisation independent of the presence and severity of COVID-19 symptoms.Research in contextEvidence before the studyAt the beginning of the study (early pandemic in April 2020), the SARS-Cov-2 specific seroprevalence was totally unknown. Additionally, S1-specific antibody assays being the first on the market were tested with limited sample size showing a lower sensitivity and specificity at that time. Furthermore, at that time, there were no unambiguous interpretations of antibody test results with regard to immunity/protection against reinfection. It was also not clear whether the detection of different antibody specificities could yield an essential input into the interpretation of the antibody’s qualities. Another open question was how long antibodies of the various specificities as well as antibodies with protective capacities would persist.Added value of this studyWe provide data to confirm the most reliable correlation of RBD-specific antibodies with neutralising antibodies that are stable for at least six months. S1- and NCP-specific antibodies wane more quickly than RBD-specific antibodies, rendering them not as ideal candidates for longitudinal seroprevalence studies. Concerning symptoms, anosmia/dysgeusia was strongly associated with NT-seropositivity and seroprotection in the overall study population.Implications of all the available evidenceOur data suggest that RBD-specific total antibody measurements with assays of high specificity can be used for cross-sectional as well as longitudinal seroepidemiological studies, even in low-prevalence settings. Detection of these antibodies also indicates robust seroprotection for at least six months. Due to the substantial loss of S1- and NCP-specific antibodies within the first months, assays targeting these antigen specificities – in contrast to RBD-specific antibody measurements – are not optimal to assess the duration of seroprotection. Overall, respiratory symptoms alone were not useful in predicting a past infection with SARS-CoV-2. However, anosmia/dysgeusia appeared to be a significant diagnostic marker, in particular for mild COVID-19.