Progressive vaccinia: A severe complication of smallpox vaccination that occurs because of an immune defect in the vaccinated individual or in a susceptible contact of that person. In progressive vaccinia, the primary vaccination site does not heal but rather expands with extensive necrosis (death of tissue), eventually covering large portions of body with extensive destruction of normal tissue. If allowed to progress, the patient may experience dire complications including toxic or septicemic shock, disseminated intravascular coagulation, superimposed infection, and bacteremia or septicemia. They may die. Aggressive use of vaccinia immune globulin (VIG) is the mainstay of treatment.