ACE2:S neutralization assay
RBD
The neutralizing antibody response was analyzed at days 8, 28, and 42 using a S-ACE2:S blocking assay which has an ECL readout on the MSD platform. The assay measures antibodies that block the binding ofACE2 to the SARS-CoV-2 S protein and RBD antigens. Blocking antibodies bind to S protein in the plate and prevent the binding of ACE2 labeled with MSD-Sulfotag. A reference standard, included in the kit, was used to generate a standard curve. Neutralizing antibody concentrations in samples were calculated by backfitting the measured signals for samples to the standard curve. Correcting for dilution provided the final neutralization antibody concentration in undiluted samples. In this assay, 1 unit/mL corresponded to neutralizing activity of 1000 ng/mL of antibody that can bind to S protein.