highly significantP < 0.001
Two weeks after the booster immunization, the serum antibody titer of the mice in the 10 μg RBD-Alum-CpG group was 5.12 × 10 6 , the serum antibody titer of the mice in the 2.5 μg RBD-Alum-CpG group was 1.08 × 10 6 , and the serum antibody titer of the mice in the 10 μg RBD-Alum group was 1.06 × 10 4 , and the difference in the antibody titers between the three groups was highly significant ( P < 0.001 ) ( Fig. 2 c–e).