Young (3-month-old) and aged (14-month-old) BALB/c mice were immunized intramuscularly on days 0 and 14 with 10 μg of monomeric SARS-CoV-2 RBD protein derived from wild type (WT-RBD), B.1.351 beta variant (B.1.351-RBD), or a 1:1 mixture (Bivalent-RBD) with indicated adjuvants. Serum samples were collected on day 28. (A and B) Anti-WT and B.1.351-RBD binding ELISA were assessed in young (A) and aged (B) mice. n=10 per group. Box-and-whisker plots represent the minimum, first quartile, median, third quartile, and maximum value. (C and D) Pseudovirus neutralizing titers (NT50) values against wild-type (circle) and B.1.351 (triangle) variant SARS-CoV-2 strains were assessed. n=5 per group. Fold differences in titers are shown. The numbers indicate GMT. Each symbol represents an animal. Data were analyzed by one-way ANOVAs corrected for multiple comparisons. **P <0.01, **** P <0.0001. LLD, lower limit of detection.