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Attenuation of mutant viruses, as determined by survival of mice. Mice were inoculated intranasally with purified virus preparations containing 5 × 106, 5 × 105, 5 × 104, or 5 × 103 PFU of wild-type vaccinia virus (WR) or mutant WR.c1 or WR.c3. Mice were weighed daily, and those that had lost more than 30% of their initial weight were sacrificed. The survival of mice was determined for 14 days after inoculation.
