Vaccinia virus is serologically related to smallpox, although its exact origin is unknown. Some virologists believe it is a recombinant virus derived from smallpox and cowpox, and others suggest it originated from a poxvirus of horses.
The virus is usually propagated by dermal inoculation of calves, and the resultant vesicle fluid (“lymph”) is lyophilized and used as a live virus vaccine in humans. The vaccine is inoculated into the epidermis and produces a localized lesion, which indicates successful immunization. The lesion becomes vesicular, then pustular, followed by crusting and healing over 10 to 14 days. The local re-action is sometimes severe and accompanied by systemic symptoms such as fever, rash, and lymphadenopathy. Patients who are immunocompromised may experience severe reactions, such as progressive vaccinia. Vaccinia-produced immunity to smallpox wanes rapidly after 3 years, and the duration of long-term immunity beyond that time is uncertain.
There has been a resurgence of scientific interest in vaccinia as a possible vector for active immunization against other diseases, such as hepatitis B, herpes simplex, and even human immunodeficiency virus. It has been shown that gene sequences coding for spe-cific immunogenic proteins of other viruses can be inserted into the vaccinia virus genome, with subsequent expression as the virus replicates. For example, a recombinant vaccinia strain carrying the gene sequence for hepatitis B surface antigen (HbsAg) can in-fect cells, lead to production of HbsAg, and stimulate an antibody response to it. Theoret-ically, gene sequences coding for a variety of antigens could be packaged in a single viable vaccinia virus, thus allowing simultaneous active immunization against multiple agents. It has been suggested that use of other poxviruses of animal or avian orgin, such as canarypox, may be even safer, yet effective vectors for use in humans. Whether such approaches become routinely applicable to clinical medicine remains to be seen.
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