CD8A and infection: (Figure 2(a) shows results for cells harvested 5 days following A/PR8 challenge in all groups: AdC68NP primed only, X31 boost only, AdC68NP followed by A/X31, and naïve) By day 20 after A/PR8 infection, numbers of NP-specific CD8+ T cells increased further in blood of AdC68NP vaccinated young mice and in naïve mice of both age groups and remained stable or decreased as compared to day 5 in the aged.