rVVs expressing IFN-γ are attenuated for immunodeficient and immunocompetent mice. (A) Clearance studies in VV-infected immunodeficient mice. No viral replication was observed in ovaries (Upper) and spleen (Lower) of mice inoculated with the rVVs expressing IFN-γ, unlike control viruses (*, P < 0.05, Mann–Whitney test). (B) Pathogenicity studies of VV-infected immunocompetent mice. Viral titers were undetectable in the ovaries of mice inoculated with rVVs v50ΔB13RMγ and v50ΔB22RMγ (*, P < 0.005, Mann–Whitney test). For A and B, viral titers are given as pfu per organ. Bars indicate mean titer per group. (C) Survival studies in BALB/cBy nude mice. Statistical analysis of survival time (log-rank test) yielded P < 0.0001 (*) for v50ΔB13R or v50ΔB22R vs. v50ΔB13RMγ or v50ΔB22RMγ.