Clonal expansion is a hallmark of adaptive immunity and has been challenging to investigate in humans in a standardized manner compared with animal models. We studied a cohort of 29 healthy individuals who received three mRNA vaccinations against SARS-CoV-2 before a breakthrough infection. We characterized the magnitude, phenotype, and clonal composition of CD8 T cell responses against 16 epitope specificities by ELISpot; flow cytometry; and single-cell RNA, protein, and T cell receptor (TCR) sequencing. One hundred six TCRs from five epitope-specific repertoires were reexpressed and tested for peptide sensitivity. Whereas vaccination-recruited T cell repertoires were enriched for high-avidity TCRs, differential clonal expansion was not linked to fine avidity differences. Instead, maintenance of polyclonality ensured robustness in counteracting viral mutational escape through altered epitopes. Deciphering the functionality of human antigen-specific T cell repertoires instructs our understanding of human T cell biology and may guide the development of vaccines and other immunotherapies.