SARS-CoV-2(8) Mutation(8) PiCoVacc(5) Human(5) reduction in(5) delta variant(5) Neutralizing activity(5) B.1.1.529(5) Delta variants(5) lead(5) geometric mean titer(5) antibody(5) the spike protein(5) in vitro(5) GMT(5) Protein(5) Omicron variant(5) inactivated(5) dose(5) GMTs(5) collected(5) variant(5) cross-neutralize SARS-CoV-2(5) immune(5) Inactivated vaccine(5) wild type(5) cross-neutralize(5) Vaccine(5) geometric mean(5) ZF2001(5)