Based on indirect ELISA data, mAb 1A9 has the strongest binding to S protein when compared with the other three mAbs. Hence, a sandwich ELISA was performed to determine if it can be paired with the human mAb CR3022 which is known to bind to the S1 subunit of SARS-CoV-2. As shown in Figure 4A, recombinant S protein was detected at 15.6 ng/mL and above when 1A9 was used as a capture antibody and CR3022 was used as a detector antibody. Since the S protein was His-tagged, a His-tagged haemagglutinin (HA) protein of influenza A virus was used to check for specificity of binding. The absorbance readings in the presence of S protein were significantly higher than that in the presence of HA for protein concentrations of 15.6 ng/mL and above.