Anti-RBD antibody response in wild-type BALB/cJ mice. (a) ELISA OD values for IgG antibody from 1:100 diluted mouse sera after a single inoculation using PBS (condition 1), sRBD (0.48 μg, condition 2), pRBD1 (condition 3), and pRBD2 (condition 4) containing 0.48 μg of RBD. Sera collected on both Day 5 and Day 12 after immunization were indicated. (b) Neutralization of HIV-1 virions pseudotyped with SARS-CoV-2 S protein by mouse sera collected on Day 12 post a single injection. Throughout the two panels, each data point in the figure represents the mean values obtained from four mice of each group. Error bars represent the standard errors. Statistical difference between sRBD and pRBD was determined by Student’s t-test (***p-value < 0.001).