Measurement of RBD binding antibodies, RBM binding antibodies, and non-RBM binding antibodies in vaccinee plasma by BLI

The SA biosensors immobilized with biotin-labeled RBD were incubated with plasma and the signal represented RBD binding antibodies. To further determine the RBM binding antibodies and non-RBM binding antibodies, the competition assay was performed. The SA biosensors loaded with RBD were immersed into 200 nM ACE2 (Novoprotein) until the binding curve reaching saturation, confirming that the RBM of RBD was occupied by ACE2. After that, the complex was incubated 1:50 diluted plasma mixed with 200 nM ACE2. The binding signal indicated that antibodies bind to other regions on RBD except RBM region. The RBM antibodies were determined by RBD antibodies minus non-RBM antibodies. Spearman’s rank correlation test was applied to measure the correlation between plasma neutralization ID₅₀ against Omicron with the RBM antibodies or non-RBM antibodies.