SARS-CoV-2–specific cellular immune responses after vaccination of aged rhesus macaques. (A) Spike protein–specific T cell responses as measured in 40 NHP PBMC samples (20 NHPs and two time points) with an IFN-γ/IL-4 double-color ELISPOT at indicated time points. The geometric mean titer (GMT) response per group is indicated with a horizontal line. Samples with background subtracted counts below or equal to zero were set at 10 and 1 for IFN-γ and IL-4, respectively, for visualization purposes and are indicated by open symbols and the dotted line. (B) Spike protein–specific Th1 and Th2 cell responses as measured in 40 NHP PBMC samples (20 NHPs and two time points) by ICS at indicated time points. Frequency of CD4⁺CD69⁺ T cell–expressing Th1 cytokines (IFN-γ and/or IL-2 and not IL-4, IL-5, and IL-13) or Th2 cytokines (IL-4 and/or IL-5 and/or IL-13). SFU, spot-forming units. The geometric mean response per group is indicated with a horizontal line. The dotted line indicates the technical threshold. Open symbols denote samples at technical threshold.