The neutralizing antibody tilters for Delta viruses after three types of sequential immunization. (A) The neutralizing antibody tilters against HB02 virus in serum of mice at days 7/14/28 after the last dose of vaccine, with sequential immunization. (B) The neutralizing antibody tilters against Omicron virus in serum of mice at days 7/14/28 after the last dose of vaccine, with sequential immunization. (C) The neutralizing antibody tilters against Delta virus in serum of mice at days 7/14/28 after the last dose of vaccine, with sequential immunization. Error bar represents SEM from eight independent experiments (t-test, * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001).