All present strains of vaccinia virus are genetically related, although they exhibit different biological properties in the laboratory. Most strains have a wide host range in experimental animals, and all exhibit rapid growth on the chorioallantoic membrane cultures. Since the early 1980s, strains of vaccinia have been engineered to express genetic information for other viral and bacterial antigens or immunomodulatory proteins. Vaccination against smallpox is most effective by infection of the skin with vaccinia virus, followed by extension of replication to the lymph nodes and spleen, which elicits cell-mediated and humoral immune responses that provide protection against infection with variola virus. The clinical events following primary inoculation of the skin with vaccinia virus are more rapid than the course of a natural smallpox infection, and are localized rather than generalized.
Inoculation produces a papule at the vaccination site on the third day after vaccination.
This papule becomes vesicular within 2 or 3 days. As with smallpox, the vesicle soon becomes pustular, mostly as a result of the entry of polymorphonuclear cells, the influx of which is induced by the viral infection. The surrounding skin becomes much more tender than in smallpox, and the lesions reach maximum size between 8 and 12 days after infection. At this time the lymph nodes are more or less enlarged and tender, and the subject sometimes experiences mild fever and malaise. The pustule dries from the center outward to become a scab that falls off about 3 weeks after vaccination to leave a typical pitted scar.