In a vaccine trial setting, intracellular cytokine staining assays measure whether individuals respond to a vaccine. They do so by measuring how antigen-specific T–cells respond to antigen stimulation. Antigen-specific T–cells that respond to antigen express one or multiple cytokines (i.e., IFNg, IL2, TNFa, IL4). The strength of response is measured as the fraction of antigen-specific T-cells e...