Impact of adjuvants on a recombinant SARS-CoV-2 spike vaccine: Protective efficacy and recall antibody response following homologous challenge

Immune imprinting, or original antigenic sin, challenges the control of evolving severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). How the quantity and quality of pre-existing immunity modulate the recall antibody response upon re-exposure remains poorly understood. We immunized hamsters with a recombinant ancestral spike protein (S-2P) formulated with one of four distinct adjuvants to assess the impact of adjuvant-induced immunity on protective efficacy. Furthermore, we analyzed the modulatory effect of the adjuvant on the potency, breadth, and evolution of antibody responses after homologous viral challenge. We found that vaccination with Montanide ISA720-adjuvanted S-2P (S-2P:ISA720) not only induced higher initial neutralizing antibody titers and conferred stronger protection but also established a pre-existing immune state capable of cross-neutralizing the antigenically distant Omicron BA.1 variant. In contrast, aluminum hydroxide adjuvanted S-2P (S-2P:Al) elicited comparatively lower neutralizing titers and showed no cross-neutralization against BA.1. Following viral challenge, the S-2P:ISA720 group exhibited significant affinity maturation toward conserved epitopes, which markedly enhanced cross-neutralization against BA.1 without increasing neutralizing titers or affinity against the homologous strain or the antigenically related Delta variant. Conversely, the S-2P:Al group mounted a narrow, imprinting-facilitated response, characterized by boosting of strain-specific antibodies without substantial improvement in BA.1 neutralization. These findings suggest that in S-2P:ISA720-immunized animals, high-affinity antibodies mediate epitope masking of immunodominant sites, thereby redirecting responses toward subdominant conserved epitopes with cross-neutralizing potential. In conclusion, our study demonstrates that adjuvants can critically guide recall immune responses toward breadth and affinity maturation, offering a rational strategy for developing next-generation vaccines against SARS-CoV-2 and other variable pathogens.