vaccinia

Vaccinia virus is a member of the genus Orthopoxvirus within the family Poxviridae. It is an enveloped, brick-shaped virus with a large double-stranded DNA genome of roughly 186,000 base pairs encoding about 200 genes. Vaccinia replicates in the cytoplasm, forming viral factories and mature virions with a complex structure that includes a dumbbell-shaped core. The virus has a broad host range and is best known for its historical use as the basis of the live smallpox vaccine.

Originating as the vaccine used against variola (smallpox), vaccinia is related to other orthopoxviruses such as cowpox and variola but is distinct from them. The exact origin of the vaccine strains used in immunization programs is uncertain, and multiple lineages circulated in the 19th and 20th centuries. Widespread vaccination with vaccinia contributed to the eradication of smallpox in 1980.

In clinical use, the classical smallpox vaccine employed live vaccinia to provoke immunity to variola. In modern biotechnology, vaccinia-based vectors are used to express foreign antigens for vaccines and gene therapy; variants such as Modified Vaccinia Ankara (MVA) are highly attenuated and used in research and some vaccines due to improved safety.

Safety considerations are important; vaccination commonly causes local reactions and mild systemic symptoms. Severe adverse events can occur, including eczema vaccinatum, progressive vaccinia, and disseminated vaccinia, particularly in individuals with skin conditions or weakened immune systems. Transmission to close contacts is possible via lesions or contaminated objects. Vaccinia immune globulin (VIG) can treat systemic infections. Vaccination is generally contraindicated in people with severe immunodeficiency, active extensive eczema, or certain pregnancy circumstances.