Mass-standardised quantitative biophotonic assay for profiling the SARS-CoV-2 spike IgG response in recovery and vaccination cohorts.

A fully quantitative biophotonic assay for SARS CoV 2 Spike IgG antibodies has been mass-standardised against the NISTmAb human antibody to produce fully quantitative results in mg L-1. The antibody response was compared directly between two vaccinated cohorts for AstraZeneca ChAdOx1-S (AZ, n = 35) and Pfizer/BioNTech BNT162b2 (Pfizer, n = 25) with patients recovering from infection in a SARS-CoV-2 RT-PCR-positive cohort (n = 195) and pre-pandemic cohort (n = 200). The vaccine response distributions were used to quantify live-challenge protective thresholds: the 31st percentile of the AZ cohort distribution was 2.90 ± 1.10 mg L-1 compared with the 7th percentile of the Pfizer cohort distribution, 1.11 ± 1.10 mg L-1 corresponding to 69% and 93% protection. Further, the anti-SARS-CoV-2 NIBSC Verification Panel showed a separation of seropositive and seronegative samples at 1.90 ± 1.10 mg L-1. Reverse cumulative distribution curves were constructed for each cohort showing mean vaccine recovery thresholds of 1.78 ± 0.95 mg L-1 (mean ± sample standard deviation) compared with mean points of inflection at 2.82 ± 0.31 mg L-1, derived from the population analysis.