CD4 and infection: Polyfunctional CD8+ T-cell responses mirrored both neutralizing antibody and CD4+ T-cell responses with the highest polyfunctional CD8+ T-cell frequency observed in Cohort 2 (p = 0.0002 vs. non-Omicron variant infection; p < 0.001 vs. triple-vaccinated transplant recipients), with responses comparable to healthcare worker triple-vaccinated controls (cohort 4) (p > 0.99).