SARS Coronavirus-2 Microneutralisation and Commercial Serological Assays Correlated Closely for Some but Not All Enzyme Immunoassays

Abstract

Serological testing for SARS-CoV-2-specific antibodies provides important research and diagnostic information relating to COVID-19 prevalence, incidence and host immune response. A greater understanding of the relationship between functionally neutralising antibodies detected using microneutralisation assays and binding antibodies detected using scalable enzyme immunoassays (EIA) is needed in order to address protective immunity post-infection or vaccination, and assess EIA suitability as a surrogate test for screening of convalescent plasma donors. We assessed whether neutralising antibody titres correlated with signal cut-off ratios in five commercially available EIAs, and one in-house assay based on expressed spike protein targets. Sera from recovered patients or convalescent plasma donors who reported laboratory-confirmed SARS-CoV-2 infection (n = 200), and negative control sera collected prior to the COVID-19 pandemic (n = 100), were assessed in parallel. Performance was assessed by calculating EIA sensitivity and specificity with reference to microneutralisation. Neutralising antibodies were detected in 166 (83%) samples. Compared with this, the most sensitive EIAs were the Cobas Elecsys Anti-SARS-CoV-2 (98%) and Vitros Immunodiagnostic Anti-SARS-CoV-2 (100%), which detect total antibody targeting the N and S1 antigens, respectively. The assay with the best quantitative relationship with microneutralisation was the Euroimmun IgG. These results suggest the marker used (total Ab vs. IgG vs. IgA) and the target antigen are important determinants of assay performance. The strong correlation between microneutralisation and some commercially available assays demonstrates their potential for clinical and research use in assessing protection following infection or vaccination, and use as a surrogate test to assess donor suitability for convalescent plasma donation.

SARS-CoV-2 neutralizing antibodies: Longevity, breadth, and evasion by emerging viral variants

“…After 72 hours, cells were stained with NucBlue (Invitrogen, USA), and each entire well was imaged with InCell Analyzer. Nuclei counts, proxy for cytopathic effect, were compared between convalescent sera, mock controls (defined as 100% neutralization), and infected controls (defined as 0% neutralization) using the formula; % viral neutralization = (D-(1-Q))x100/D, where Q = nuclei count normalized to mock controls and D = 1-Q for average of infection controls (InCarta software) (S8D and S8E Fig) . The cutoff for determining the neutralization endpoint titer of diluted serum samples was set to �50% neutralization (S8F Fig), and the cutoff of positivity for neutralization on live virus was 1:40 [51]. During assay validation, 5 out of the 10 NIBSC standards (CS678 WHO protocol) were positive in the assay with mid and high levels of live neutralization, consistent with the confirmed results from the NIBSC.…”

Convalescent plasma in patients admitted to hospital with COVID-19 (RECOVERY): a randomised controlled, open-label, platform trial

“…Convalescent donors were chosen based on high anti-spike IgG concentrations, using an ELISA that has been shown to correlate well with neutralising antibody. 22 , 23 , 24 We used a EUROIMMUN sample to cutoff ratio of 6·0 for plasma to qualify for use in this trial, which is substantially more than the 3·5 cutoff that the US FDA recognises as high titre. 9 Nearly all participants received plasma from two different donors to increase the chance that at least one contained higher concentrations of neutralising antibodies.…”

“…EUROIMMUN IgG has been shown to correlate well with neutralisation assays, and a sample to cutoff ratio of 6·0 or more was previously shown to be associated with neutralising titres of 1:100 or more in convalescent plasma. 21 , 22 , 23 , 24 The US FDA has determined that convalescent plasma with a EUROIMMUN sample to cutoff of 3·5 or more qualifies as high titre and can be used for the treatment of hospitalised patients under an Emergency Use Authorisation. 9 Patients in the convalescent plasma group received two units (275 ml [200–350]) intravenously, the first as soon as possible after randomisation and the second (from a different donor) the following day and at least 12 h after the first.…”

Passive Immunity Should and Will Work for COVID-19 for Some Patients

“…Anti-S antibody levels correlate well with those of neutralizing antibodies, including anti-RBD [30,33,38,48,62,64,81,86,88,90,106,[126][127][128][129][130][131][132]. Piccoli et al [50] have proposed that some 90% of neutralizing activity is due to anti-RBD antibody, and they have particularly mapped two dominant subepitopes.…”