[Capacity of clones of vaccinia virus to multiply at different temperatures and pH values].

The study of the biological characteristics of ten clones, isolated from plaque, of vaccinia virus has made evident that two of these characteristics--the capacity to multiply at 40 degrees C and the type of CPE in animal cell cultures--can be used as genetic markers of neuropathogenicity, valued in relationship to the property to induce demyelinating encephalitis in mice injected intracerebrally or intravenously. The most neurovirulent clones demonstrated high capacity to multiply at 40 degrees C and lytic CPE; the clones, that did not have (or had at a very low level) encephalitogenic activity in mice, presented poor capacity to multiply at 40 degrees C and CPE of syncytial type. The variation of the pH of the cell culture medium enhances the difference of CPE induced by clones that had different levels of neuropathogenicity. The results are discussed in relation to their use in improving the smallpox vaccine.