IFNG and viral infectious disease: Seven days after the first vaccine dose, we observed that all immunization groups, i.e., single recombinant virus vaccines, Formulation 1 and YF 17DD vaccine control were not able to induce a considerable response of IFN-γ spot forming-cells (SFC) specific to yellow fever since there were no statistically differences among the number of IFN-γ secreting splenocytes between vaccinated groups and the mock group (Figure 5A).