Summary: The emergence of the SARS-CoV-2 Omicron BA.1 (B.1.1.529) variant has raised questions regarding resistance to neutralizing antibodies elicited by natural infection or immunization. We examined the neutralization activity of sera collected from previously SARS-CoV-2-infected individuals and SARS-CoV-2 naive individuals who received BBIBP-CorV or CoronaVac to BA.1 and the earlier variants Alpha, Beta, and Delta. Both sera from convalescent patients over three months after infection and two-dose BBIBP-CorV or CoronaVac vaccine recipients barely inhibited BA.1, less effectively neutralized Beta and Delta, and moderately neutralized Alpha. However, administering a single dose of BBIBP-CorV or CoronaVac in previously infected individuals or a third dose booster vaccination of BBIBP-CorV or CoronaVac in previously vaccinated individuals enhances neutralizing activity against BA.1 and other variants, albeit with a lower antibody titer for BA.1. Our data suggest that a booster vaccination is important to broaden neutralizing antibody responses against the variants.
