Efficient boosting of Omicron-reactive memory B cells after breakthrough infection protects from repeated exposure.

Exploring the impact of persistent mutations in SARS-CoV-2 variants and reduced immunity on breakthrough infections (BTIs) is crucial, particularly in understanding how antigen-specific memory B cells (MBCs) respond to new variants. We followed 107 participants who received the ancestral inactivated vaccine and experienced one or two Omicron BTIs over six months. Using flow cytometry, SARS-CoV-2 antigen probes, single-cell RNA sequencing, and B cell receptor (BCR) profiling, we assessed MBCs and immune diversity. Our findings revealed that although neutralizing antibody levels decreased over time, the number of specific MBCs remained stable and matured progressively. Notably, pre-existing Omicron-specific MBCs played a key role in preventing secondary Omicron infections. Differential gene analysis showed enrichment in antigen processing and immune regulation pathways, while clonal lineage analysis revealed more B cell expansion and V(D)J gene-specific rearrangements in high neutralization samples. These results emphasize MBCs' critical role in long-term immunity and inform future vaccination strategies.