ABSTRACT
The spike glycoprotein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) continues to accumulate substitutions, leading to breakthrough infections of vaccinated individuals and prompting the development of updated booster vaccines. Here, we determined the specificity and functionality of antibody and B cell responses following exposure to BA.5 and XBB variants in individuals who received ancestral SARS-CoV-2 mRNA vaccines. BA.5 exposures elicited antibody responses that primarily targeted epitopes conserved between the BA.5 and ancestral spike, with poor reactivity to the XBB.1.5 variant. XBB exposures also elicited antibody responses that targeted epitopes conserved between the XBB.1.5 and ancestral spike. However, unlike BA.5, a single XBB exposure elicited low levels of XBB.1.5-specific antibodies and B cells in some individuals. Pre-existing cross-reactive B cells and antibodies were correlated with stronger overall responses to XBB but weaker XBB-specific responses, suggesting that baseline immunity influences the activation of variant-specific SARS-CoV-2 responses. HighlightsO_LIVariant breakthrough infections boost ancestral cross-reactive antibodies and B cells C_LIO_LIFirst and second BA.5 exposures fail to elicit variant-specific antibodies and B cells C_LIO_LIXBB infections and monovalent vaccinations elicit XBB.1.5-specific responses in some individuals C_LIO_LIXBB.1.5-specific responses correlate with low levels of pre-existing humoral immunity C_LI