Abstract
Background: During New Zealand’s first outbreak in early 2020 the Southern Region had the highest per capita SARS-CoV-2 infection rate. PCR testing was initially limited by a narrow case definition and limited laboratory capacity, so cases may have been missed. Serological assays that detect SARS-CoV-2 antibodies are an important tool for determining past infection and investigating immune responses. However, it was not known how long the antibodies persist.
Objectives: Four main aims relating to SARS-CoV-2 serological testing were explored: 1) evaluate the clinical performance and concordance of seven serological assays, including the surrogate virus neutralization test, which estimates the level of neutralising antibodies; 2) determine the frequency of SARS-CoV-2 antibodies in higher-risk individuals in the Southern Region, to determine whether any cases were missed during the first outbreak; 3) assess the likelihood of infection among those diagnosed as ‘probable’ cases, using serological testing; and 4) assess the persistence of SARS-CoV-2 antibodies in recovered COVID-19 patients.
Study design: Pre-pandemic sera (n=300) were used to establish assay specificity and sera from PCR-confirmed SARS-CoV-2 individuals (n=78) to establish sensitivity. All higher-risk participant sera (n=1127) were tested on the Abbott Architect assay, and any samples with grey-zone or positive results were tested on five additional assays. Sera from the probable cases (n=9) were tested on all seven assays. Results run on multiple assays were used for the assessment of assay concordance. For the antibody persistence study, sera from PCR-confirmed COVID-19 patients (n=42) collected at multiple time points, up to 10-12 months post-infection were tested on four assays.
Results: The median time from infection onset to serum collection for the first time-point for the PCR-confirmed cases was 14 weeks (range 11-17 weeks). The nucleocapsid based Abbott Architect IgG assay demonstrated suboptimal sensitivity (76.9%). The nucleocapsid-based Roche pan-Ig assay demonstrated highest sensitivity (100%). The spike-based assays demonstrated high sensitivities ranging 89.7-94.9%. Anti-RBD IgG antibodies had the strongest correlation with NAbs (kappa statistic = 0.914; Pearson’s r = 0.914). Nine previously undiagnosed seropositive individuals were identified within the higher-risk individuals and all had epidemiological risk factors. Seven of the nine probable cases were negative on all the assays, suggesting that these individuals likely did not have infection. The sensitivity of the Abbott Architect assay dropped to 15.4% by the last time point (median of 45 weeks post-infection), while the S-based assays remained above 80%.
Conclusions: The results demonstrated SARS-CoV-2 antibodies measured on different assays using different antigen targets and different assay formats can give varying results and highlights the need to choose an assay appropriate for the testing purpose and to consider an orthogonal testing algorithm to reduce false positive and false negative results. Importantly, in this study there was extremely low probability the PCR confirmed cases had immune boosting from re-exposure to SARS-CoV-2, thus the antibody responses measured can be attributed to the initial exposure. Significantly, no unexpected undiagnosed SARS-CoV-2 infections were found in the Southern region of NZ, supporting the elimination status of the country at the time this study was conducted.