Evaluation of two SARS-CoV-2 immunoassays

Master Thesis


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Aim: The purpose of this study is to verify the performance of the Roche Elecsys ® antinucleocapsid (qualitative) and anti-spike (quantitative) SARS-CoV-2 immunoassays to determine whether the performance of the assays is acceptable for diagnostic use in the Groote Schuur Hospital virology/chemistry laboratory, as well as other National Health Laboratory Service (NHLS) laboratories in South Africa. Methods: We performed a verification study using de-identified remnant serum or plasma samples. Standard verification experiments including sensitivity, specificity and precision were performed. Pre-pandemic samples were used to assess specificity. Samples with a linked positive SARS-CoV-2 polymerase chain reaction (PCR) result on a respiratory sample >10 days before the serum/plasma collection date were used to assess sensitivity. Additionally, postvaccine humoral response and other parameters was assessed in a cohort of laboratory staff. Results: For the anti-nucleocapsid antibody assay, specificity was 99.7% based on 316 samples and sensitivity 91.3% based on 404 samples. For the anti-spike antibody assay, the specificity based on 194 samples was 100%, and the sensitivity based on 384 samples was 93.8%. Both assays demonstrated acceptable precision. Furthermore, the anti-spike antibody assay sensitivity was >92% during the first three waves in South Africa, dominated by different SARS-CoV-2 variants. Post-vaccine seroconversion in 115 staff with no evidence of prior natural infection was 99% and hybrid immunity produced higher anti-spike antibody titres compared to vaccine-only participants. Conclusion: Both immunoassays met our acceptance criteria. Both assays can be used for seroprevalence studies. The anti-nucleocapsid immunoassay assay is valuable in confirming past natural infection in patients with previous asymptomatic infection, previous symptomatic infection where no PCR was done or PCR-negative patients who present to hospital with COVID-19 during the second week of illness or later. Most importantly, the antispike immunoassay can be used as a reliable, cheap, and easily accessible surrogate marker of post-vaccine humoral immune response and we recommend using this to confirm and monitor humoral immune response in patients with risk factors for non-seroconversion following vaccination and increased risk for morbidity and mortality following infection with SARS-CoV-2.