Mapping SARS-CoV-2 immunity after an XBB.1.5 booster by antigenic cartography of merged human and hamster sera.

The ongoing evolution of SARS-CoV-2, particularly the emergence and rapid spread of new immune-evasive variants, continues to challenge the durability of vaccine-induced protection. Understanding how repeated variant exposures shape neutralizing antibody breadth is therefore essential for optimizing booster design. Here, we investigated polyclonal neutralizing antibody responses in individuals who received a bivalent (ancestral + BA.4/5) boost followed by an additional monovalent XBB.1.5 boost, with and without breakthrough infection, against a diverse panel of SARS-CoV-2 variants. To visualize human multi-exposure immunity in antigenic space via antibody landscapes, we extended our existing human sera-based antigenic map with hamster sera infected with more recent variants. The hamster sera allowed us to map BA.2.86 and JN.1 variants, which largely escape human single exposure sera. Our analysis of human multi-exposure sera revealed that the number and type of exposures significantly shaped antibody landscapes. The XBB.1.5 booster immunization notably increased neutralizing antibody titers across variants, elevating the height of the antibody landscape. However, titers against more recent Omicron variants, such as JN.1, were low despite booster administration. These findings highlight the dynamic nature of SARS-CoV-2 immunity and emphasize the need for continuous monitoring and adaptation of vaccine strategies to maintain effective protection against emerging variants.