Highlights An extensive evaluation of the Abbott SARS-CoV-2 IgG assay. The assay shows excellent sensitivity/specificity with little cross-reactivity. Healthy volunteers and pre-pandemic samples had similar antibody cut-off indexes. A lower, optimised limit for reactivity improves sensitivity in early disease.

Background: Subjects with previous COVID-19 have augmented post-vaccination responses. However, the antibody response in COVID-naïve subjects from Southeast Asia is not well known. Methods: 77 COVID-naïve vaccinees were tested with a full antibody panel [spike antibodies (total (T-Ab), IgG, IgM) and neutralizing antibodies (N-Ab)] pre-vaccination, 10 days after dose 1, and 20/40/60/90/120/150/180 days after dose 2. Results: 10 days after dose 1, 67.6% (48/71)/69.0% (49/71) were T-Ab/IgG positive; only 15.5% (11/71)/14.1% (10/71) were N-Ab/IgM positive. While all (100%) subjects had brisk T-Ab, IgG and N-Ab antibody responses 20 days after complete vaccination, only 79.1% (53/67) were IgM positive. At 180 days (n = 8), T-Ab/IgG/N-Ab were still reactive (lowest T-Ab 186 U/mL, IgG 617 AU/mL, N-Ab 0.39 µg/mL), but IgM was negative in all samples. Spike antibody thresholds of T-Ab 74.1 U/mL (r = 0.95) and IgG 916 AU/mL (r = 0.95) corresponded to N-Ab reactivity (>0.3 µg/mL). Non-linear regression analysis showed that N-Ab would decrease to 0.3 µg/mL by 241 days, whereas T-Ab/IgG would need 470/163 days to reach titers of T-Ab/IgG associated with a N-Ab 0.3 µg/mL (76.4 U/mL and 916 AU/mL respectively). Conclusions: The antibody responses of T-Ab, IgG and N-Ab remain high and durable even at 180 days. N-Ab titers are expected to remain reactive up to 241 days post-vaccination.

Introduction: We tested the total spike antibody (S-Ab), IgG/IgM S-Ab, and neutralizing antibody (N-Ab) responses of COVID-19-naïve subjects from before their first BNT162b2 vaccination up to 210 days after boosting. Methods: We studied 136 COVID-19-naïve subjects who received three doses of the Pfizer mRNA vaccine (39 males, 97 females, mean age 43.8 ± 13.5 years) from January 2021 to May 2022. Serum was assessed for total S-Ab (Roche), IgG/M (Abbott), and N-Ab (Snibe). Results: Peak antibody levels were measured 20-30 days after each dose, with booster dosing eliciting significantly higher peak antibodies than the second dose: total S-Ab 2219 vs. 19,551 BAU/mL (difference 16,667 BAU/mL, p < 0.0001); IgG 2270 vs. 2932 BAU/mL (difference 660 BAU/mL, p = 0.04); and N-Ab 3.52 vs. 26.4 µg/mL (difference 21.4 µg/mL, p < 0.0001). Only IgM showed a lower peak post-booster antibody titer (COI 2.11 vs. 0.23, difference 1.63, 95% CI 1.05 to 2.38, p < 0.0001). By 180–210 days after the second or third vaccination, total S-Ab/IgG/N-Ab had decreased by 68.7/93.8/73.6% vs. 82.8/86.3/79.5%. The half-lives of IgG and N-Ab antibodies were longer after the third vaccination (IgG: 65 vs. 34 days, N-Ab: 99 vs. 78 days). Conclusion: Total S-Ab/IgG/N-Ab showed a greater increase post-booster, with IgG/N-Ab having a longer half-life.

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Introduction: Antibodies to the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can increase as soon as 10-13 days after infection. We describe our evaluation of the Abbott SARS-CoV-2 IgG assay on the Architect immunoassay analyser. Methods: We assessed the precision, sensitivity, and specificity of the Abbott SARS-CoV-2 IgG assay in samples from polymerase chain reaction (PCR) positive patients and healthy healthcare workers. The manufacturer cut-off index (COI) of 1.4 was adopted to identify positive results. We examined the assay cross-reactivity with other viral antibodies (influenza/dengue/hepatitis C/hepatitis B) and rheumatoid factor (RF). The sample throughput of the Abbott assay was also assessed. Results: The Abbott assay showed excellent precision, with a CV of 3.4% for the negative control (COI = 0.06) and 1.6% for a high positive serum sample (COI = 8.6). Residual serum was available from 57 inpatients not initially suspected of having COVID-19, 29 of whom tested positive for SARS-CoV-2 IgG. The Abbott assay has a sensitivity of 90.9-100% when tested in 54 subjects ≥14 days post PCR positive, and a specificity of 100% (N = 358). There was no cross-reactivity with other viral antibodies (influenza/dengue/hepatitis C/hepatitis B) and RF. The Architect Abbott assay has a throughput of 100 samples in 70 minutes. Conclusion: The Abbott SARS-CoV-2 IgG assay shows excellent performance that is well within FDA and CDC guidelines when testing patients ≥14 days POS with little cross-reactivity from other viral antibodies. There is some evidence that SARS-CoV-2 IgG develops early in the disease process.