Omicron is the evolutionarily most distinct severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) to date. We report that Omicron BA.1 breakthrough infection in BNT162b2-vaccinated individuals resulted in strong neutralizing activity against Omicron BA.1, BA.2, and previous SARS-CoV-2 VOCs but not against the Omicron sublineages BA.4 and BA.5. BA.1 breakthrough infection induced a robust recall response, primarily expanding memory B (B _MEM ) cells against epitopes shared broadly among variants, rather than inducing BA.1-specific B cells. The vaccination-imprinted B _MEM cell pool had sufficient plasticity to be remodeled by heterologous SARS-CoV-2 spike glycoprotein exposure. Whereas selective amplification of B _MEM cells recognizing shared epitopes allows for effective neutralization of most variants that evade previously established immunity, susceptibility to escape by variants that acquire alterations at hitherto conserved sites may be heightened.