FIG. 2.

Sensitivity of candidate LAIV delNS1 and wild-type (Wt) viruses to the action of IFN. Following the exposure of 1-day-old monolayers of primary CEKs to rChIFN-α for 24 h at 38.5°C, the cell monolayers were challenged with PFPs of the respective viruses. An overlay of agarose in medium lacking serum or trypsin was incubated for 3 days before the surviving fraction of PFPs was determined. All viruses revealed biphasic survival curves, with the highest sensitivity occurring at low doses of IFN and a more resistant fraction being found at higher doses. The vac⁺ pc3 and pc4 variants, along with the wild-type virus, were experimentally indistinguishable (upper curve) and were less sensitive than the vac⁻ pc1 and pc2 variants (lower curve). Error bars represent the standard deviations of two determinations. PR₅₀, the amount of IFN that reduces the PFP titer by 50%.