Anti-S and anti-RBD MSD assay
The responses induced by PTX-COVID19-B on anti-COVID-19 S-protein IgG and anti-COVID-19 RBD IgG were analyzed from serum samples using a multiplex immunoassay sandwich-based method with an electrochemiluminescent (ECL) readout on the MSD platform. Diluted samples as well as calibrators and controls were added to MSD plates (multi-spot wells) coated with either the S or RBD SARS-CoV-2 antigens to capture human IgG antibodies present in the samples. Bound IgG antibodies were then detected using sulfo-TAG labeled anti-human IgG antibody. The human IgG antibody concentration was determined by interpolation of the ECL count on the calibration curve. The assay was validated and performed under GLP conditions at CIRION (Laval, Quebec; V-PLEX® SARS-CoV-2 Panel 2 Kit). Analysts knew timepoints but not dose levels or regimen of the samples. To convert the results from MSD arbitrary units (AU/mL) to the World Health Organization (WHO) International Standard in binding antibody units (BAU)/mL, a conversion factor of 0.0272 was applied for anti-RBD IgG concentrations and 0.00901 for anti-S IgG concentrations. Additionally, anti-S and anti-RBD IgG measurements for 21 control serum samples from convalescent patients with a documented positive SARS-CoV-2 RNA diagnosis (mentioned in the test as Human Convalescent Sera or HCS) were also performed (Supplementary Fig. 