To measure both the antigen-dependence, as well as the kinetics of Blimp1/ICOS expression during viral skin infection, we quantified the frequency of effector P14 CD8+ T cells co-expressing ICOS and Blimp1 (ICOS+Blimp1+) at different time points following VacV skin infection (+/- Ag). Here, ICOS is linked to skin infection.