CASP3 and infection: In contrast to IFN-deficient Vero cells, in which WT or delNS1 PR8 virus infection increased caspase 3/7 activity by approximately 6-fold at 36 and 48 hpi (Figure 6C), infection of A549 cells with WT or delNS1 virus at the same MOI (an MOI of 1) increased the activity less than 1.5-fold at 36 and 48 hpi (Figure 8A), suggesting that the type I IFNs inhibit apoptosis in infected cells.