/The three anti-sera samples from immunizations were used to determine if they contained antibodies that could neutralize the ability of RBD binding to ACE-2 using a commercially available surrogate viral neutralization assay kit. In this assay, HRP-labeled RBD is incubated with dilutions of anti-sera, and this mixture is added to plates coated with ACE-2. Data is recorded as percent signal inhibition. Only anti-sera from rRBD mice inhibited the binding of RBD to ACE-2, and this ability was at the maximal level detected by the kit (Figure 4).