SARS-CoV-2 has evolved into several genetic variants, all bearing mutations that reduce antibody binding and affect vaccine and treatment effectiveness. Updated COVID-19 vaccines, including bivalent formulations (wild type [WT]/BA.1 or WT/BA.5) and more recent monovalent versions targeting emerging variants such as XBB.1.5, JN.1, KP.2 or LP.8.1, were developed to broaden protection. However, immune imprinting may limit the induction of neutralizing antibodies against strains that differ significantly, even after receiving several variant-specific boosters. A deeper understanding of how booster vaccination reshapes antibody specificity remains essential for rational vaccine design. We examined the antibody response to a bivalent WT/BA.5 booster, focusing on antibody levels and neutralization. Serum samples collected before and after a fourth dose of monovalent WT or bivalent (WT/BA.5) mRNA vaccines were compared with sera from individuals after primary WT infections and Omicron BA.1, BA.2, or BA.5 breakthrough infections. We found that both monovalent and bivalent boosters significantly increased IgG and neutralizing antibodies, but breakthrough infections induced broader cross-reactive responses. Depletion experiments revealed that booster-induced immunity was predominantly mediated by cross-reactive antibodies, with the highest levels after breakthrough infections and the lowest after a primary WT infection. These findings provide functional insights into the antibody specificities associated with imprinting effects following variant-adapted booster vaccination.
