Frequency of detection of BK virus–specific CD8 and CD4 T cells across time points. (A) Any BK virus–specific T cells, in cases and controls. (B) BK virus–specific IFN-γ⁺, IL-2⁺, TNF-α⁺, and cytolytic T cells, in cases and controls. (C) Frequency of detection of BK virus–specific IFN-γ⁺, IL-2⁺, TNF-α⁺, and cytolytic CD8 and CD4 T cells across time points, in cases with a single episode of BK virus reactivation and cases with recurrent symptoms. *P < .05, **P < .01, ***P < .001, Pearson’s χ² test (A) and independent-samples Mann-Whitney U test (B-C).