In vitro infection of PBMCs isolated from vaccine-naïve study volunteers (n = 16) before vaccination with either of the two vaccine strains (Figure 4A) indicated that Ty21a displayed greater capacity to induce surface expression of NK cell functional activation markers, including CD25 (p < 0.01) and CD107a (p < 0.0001), but not intracellular production of IFN-γ (p = 0.08), compared with M01ZH09. This evidence concerns the gene IFNG and infection.