The virology of variola minor. Correlation of laboratory tests with the geographic distribution and human virulence of variola isolates.

Several groups of variola isolates were compared in DNA structure, and by four independent biologic markers. Isolates of variola minor from Europe and South America (alastrim virus) could be distinguished from African isolates of variola minor by DNA structure and by two of the four biologic markers. Taken as a group, the properties of African isolates, in general, differed from those of variola major, but this difference was confined to properties which depended (in the laboratory) on the recent history of the virus concerned. The suggestion made previously that there was an "intermediate" or "African" variety of variola virus is discounted. Laboratory tests did not distinguish any individual African isolate from variola major virus. It is concluded that a virus which may be called "alastrim" represents a "fixed" variant of variola virus, whose distribution is consistent with the dramatic spread of variola minor through the Americas and Europe in the early part of this century, and that variola minor in Africa in recent years was due to variola virus which was not alastrim and which laboratory evidence fails to identify as an entity distinguishable from variola major virus.