The Kuwait-5-67 strain of variola virus, although indistinguishable from variola virus in its other properties, behaved atypically in immuno-diffusion tests with antivaccinia serum. The precipitation line pattern produced was similar to that of cowpox virus but lacked one of the main precipitation bands given by all other strains of variola, alastrim and vaccinia viruses tested. Trypsin treatment of cowpox virus preparations revealed the missing precipitation line but did not do so with the Kuwait-5-67 strain. The antivaccinia serum absorbed by Kuwait-5-67 strain lost its ability to interact with this strain, but still gave precipitation lines with variola, vaccinia and cowpox viruses.
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