An effective interaction between the humoral and cellular arms of the immune system is key to mounting a protective immune response. As we are working to decipher which immune cells contribute to protection against CHMI, technologies that allow measuring a large number of parameters simultaneously are critical. By assessing immune responses via Cytometry by Time-of-Flight (CyTOF), Stanford University has the opportunity to comprehensively characterize cell populations that become activated post-vaccination, with regard to surface markers and intracellular cytokine expression. The technology will be used to characterize differences in immune responses between immunization regimens and protected and non-protected subjects.