Bivalent mRNA vaccine booster enhances immunity against XBB.1.5 more effectively than breakthrough infection in K18-hACE2 mice

The rapid emergence of SARS-CoV-2 Omicron subvariants, including the highly immune-evasive XBB.1.5 lineage, continues to challenge vaccine effectiveness. We evaluated immune responses and protection against XBB.1.5 challenge in K18-hACE2 mice receiving two or three doses of a bivalent mRNA vaccine (encoding Wuhan-1 and Omicron BA.4/5 spike) or two doses followed by intranasal XBB.1.5 exposure. Homologous boosting with bivalent vaccine induced stronger neutralizing antibody and cellular immune responses than breakthrough infection against the antigenically distant XBB.1.5 variant. A two-dose series alone protected mice from severe disease, but boosting further enhanced protection, reducing lung viral RNA and inflammation. Although productive infection following intranasal exposure was not confirmed, enhanced responses and viral control suggest reactivation of immune memory. Overall, homologous boosting with the bivalent vaccine enhances cross-variant immunity and protects against XBB.1.5 in a naive mouse model, supporting the utility of updated vaccines in improving immune breadth and protection against emerging SARS-CoV-2 variants.