Abstract

With the aim of broadening immune responses against the evolving SARS-CoV-2 Omicron variants, bivalent COVID-19 mRNA vaccines that encode the ancestral and Omicron BA.5 spike proteins have been authorized for clinical use, supplanting the original monovalent counterpart in numerous countries. However, recent studies have demonstrated that administering either a monovalent or bivalent vaccine as a fourth vaccine dose results in similar neutralizing antibody titers against the latest Omicron subvariants, raising the possibility of immunological imprinting. Utilizing binding immunoassays, pseudotyped virus neutralization assays, and antigenic mapping, we investigated antibody responses from 72 participants who received three monovalent mRNA vaccine doses followed by either a bivalent or monovalent booster, or who experienced breakthrough infections with the BA.5 or BQ subvariant after vaccinations with an original monovalent vaccine. Compared to a monovalent booster, the bivalent booster did not yield noticeably higher binding titers to D614G, BA.5, and BQ.1.1 spike proteins, nor higher virus-neutralizing titers against SARS-CoV-2 variants including the predominant XBB.1.5 and the emergent XBB.1.16. However, sera from breakthrough infection cohorts neutralized Omicron subvariants significantly better. Multiple analyses of these results, including antigenic mapping, made clear that inclusion of the ancestral spike prevents the broadening of antibodies to the BA.5 component in the bivalent vaccine, thereby defeating its intended goal. Our findings suggest that the ancestral spike in the current bivalent COVID-19 vaccine is the cause of deep immunological imprinting. Its removal from future vaccine compositions is therefore strongly recommended.