Evaluation of various virulence tests with low virulence vaccinia virus in mice

Two vaccinia virus (VV) clones, denoted P20 and P21 were derived from the parental VV strain Praha. The residual virulence of these two viruses in mice was compared. Intracerebral inoculation of 21-day-old mice did not reveal any marked difference between the two viruses. Tests using intracerebral inoculation of 10-day-old mice showed that their LD50 differed by 2 log10 pfu, P20 being the more attenuated. The difference between the two viruses increased further when intranasal and intracerebral inoculations of 3-day-old mice were employed. Their LD50 differed by 4 and 5 log10 pfu, respectively. Thus, these tests proved to be the most sensitive for determining residual virulence of VV in mice and thus the most suitable for differentiating among the low virulence VVs. The antibody tests carried out in the surviving mice suggested that the more attenuated virus was less immunogenic than the more virulent virus.