Over past decades the 17DD yellow fever vaccine has proved to be effective in controlling yellow fever and promises to be a vaccine vector for other diseases, but the cellular and molecular mechanisms by which it elicits such broad-based immunity are still unclear. In this study we describe a detailed phenotypic investigation of major and minor peripheral blood lymphocyte subpopulations aimed at characterizing the kinetics of the adaptive immune response following primary 17DD vaccination. Our major finding is a decreased frequency of circulating CD19⁺ cells at day 7 followed by emerging activation/modulation phenotypic features (CD19⁺interleukin(IL)10R⁺/CD19⁺CD32⁺) at day 15. Increased frequency of CD4⁺human leucocyte antigen D-related(HLA-DR⁺) at day 7 and CD8⁺HLA-DR⁺ at day 30 suggest distinct kinetics of T cell activation, with CD4⁺ T cells being activated early and CD8⁺ T cells representing a later event following 17DD vaccination. Up-regulation of modulatory features on CD4⁺ and CD8⁺ cells at day 15 seems to be the key event leading to lower frequency of CD38⁺ T cells at day 30. Taken together, our findings demonstrate the co-existence of phenotypic features associated with activation events and modulatory pathways. Positive correlations between CD4⁺HLA-DR⁺ cells and CD4⁺CD25^high regulatory T cells and the association between the type 0 chemokine receptor CCR2 and the activation status of CD4⁺ and CD8⁺ cells further support this hypothesis. We hypothesize that this controlled microenviroment seems to be the key to prevent the development of serious adverse events, and even deaths, associated with the 17DD vaccine reported in the literature.