The testing of new lots of vaccinia vaccine made using standard methods, or of well-characterized strains of vaccinia grown in tissue culture cells, would not require live variola virus. Although not of overriding concern, retention of live variola virus stocks would, however, permit corroborating assessments of the probable efficacy of new tissue culture vaccines, using laboratory assays to measure immune inhibition of variola replication or testing against variola challenge in animal models not yet available. Variola virus would have to be available for the later phases of evaluation of any such novel vaccines before their clinical use could be considered. The commercial smallpox vaccine currently approved for use in the United States is a lyophilized preparation of live vaccinia virus prepared from calf lymph. These vaccines exhibit high seroconversion rates and infrequent adverse events.
Postvaccinal encephalitis is the most serious complication in otherwise healthy individuals.
It generally affects infants less than 1 year old, and is associated with a mortality rate of about 25 percent and a risk of permanent neurologic consequences in about 25 percent of its survivors. Finally, progressive vaccinia is a severe, potentially fatal illness that occurs almost exclusively among persons with cellular immunodeficiency. The HIV epidemic and the new immune-modulating drugs and therapies currently used to treat cancers and allow successful transplantation of organs and bone marrow have placed many people worldwide at risk of this complication. The largest problem is likely to be in countries with substantial prevalence of HIV infection. CDC maintains the vaccinia under contract with Wyeth, while Baxter-Highland Laboratories maintains VIG for the U.S. Department of Defense.