CD4+ and CD8+ T Cell and Antibody Correlates of Protection Against Delta Vaccine Breakthrough Infection: A Nested Case-Control Study Within the PITCH Study

ABSTRACTT cell correlates of protection against SARS-CoV-2 infection after vaccination (‘vaccine breakthrough’) are incompletely defined, especially the specific contributions of CD4+ and CD8+ T cells. We studied 279 volunteers in the Protective Immunity from T Cells in Healthcare Workers (PITCH) UK study, including 32 cases (with SARS-CoV-2 positive testing after two vaccine doses during the Delta-dominant era) and 247 controls (no positive test nor anti-nucleocapsid seroconversion during this period). 28 days after second vaccination, before all breakthroughs occurred, cases had lower ancestral S- and RBD-specific immunoglobulin G titres and S1- and S2-specific T cell interferon gamma (IFNγ) responses compared with controls. In a subset of matched cases and controls, cases had lower CD4+ and CD8+ IFNγ and tumour necrosis factor responses to Delta S peptides with reduced CD8+ responses to Delta versus ancestral peptides compared with controls. Our findings support a protective role for T cells against Delta breakthrough infection.