CD27 and infection: A large proportion of cross-reactive memory B cells exhibited an activated phenotype (CD21− CD27+) in PBMC samples taken ~day 10 after symptom onset (median, 58%), indicating efficient antigen recall during Omicron breakthrough infection, which later waned over the course of recovery (median, 14% at ~day 160) and transitioned to a resting memory phenotype (Fig. 3C and fig.