Vaccinia virus (VACV) is a large, enveloped, double-stranded DNA virus belonging to the genus Orthopoxvirus within the family Poxviridae, subfamily Chordopoxvirinae. It is closely related to variola virus (the causative agent of smallpox), cowpox virus, and monkeypox virus (mpox), all of which share extensive genomic homology and a degree of cross-protective immunity. The precise natural origin and reservoir host of vaccinia virus remain uncertain; it is not known to circulate naturally in the wild and exists today principally as a laboratory and vaccine virus, having been used historically, and with some strains still in use, as the live vaccine that enabled the global eradication of smallpox.

Vaccinia virions are brick-shaped or ovoid particles measuring approximately 200 to 300 nanometres, making vaccinia one of the largest and most structurally complex of all known animal viruses, visible by light microscopy. The genome is a linear, double-stranded DNA molecule of roughly 190 kilobase pairs encoding around 200 genes, with covalently closed hairpin termini that link the two DNA strands at each end. Unlike most DNA viruses, vaccinia replicates entirely within the cytoplasm of infected cells, independent of the host nucleus. To achieve this, the virus packages its own multi-subunit RNA polymerase, capping and methylating enzymes, and poly(A) polymerase within the virion core, allowing it to initiate transcription of early genes immediately upon entry. Two structurally and functionally distinct infectious forms are produced during replication, the intracellular mature virion (IMV), which is released upon cell lysis, and the extracellular enveloped virion (EEV), which acquires an additional membrane from the trans-Golgi network and mediates long-range and cell-to-cell spread.

Viral gene expression follows a tightly regulated temporal cascade of early, intermediate, and late phases, each governed by distinct promoter classes and transcription factors, and coordinated with genome uncoating, DNA replication, and virion morphogenesis, which proceeds through a series of crescent, immature, and mature particle intermediates within cytoplasmic viral factories.

Vaccinia virus was the agent used in the World Health Organization’s global smallpox eradication campaign, administered by scarification to produce a localised pustular lesion and durable protective immunity against variola virus, culminating in the certified eradication of smallpox in 1980. Several vaccinia strains were used historically for this purpose, including Lister, Copenhagen, Western Reserve, and Wyeth (New York City Board of Health). Modified vaccinia Ankara (MVA) is a highly attenuated strain, generated by extensive serial passage in chicken embryo fibroblasts, that has lost the ability to complete productive replication in most mammalian cells while retaining strong immunogenicity, making it a favoured platform for modern vaccine development.

Because of its large genome, tolerance for foreign DNA insertion, cytoplasmic replication, and well-characterised biology, vaccinia virus is one of the most widely used viral platforms in biomedical research and vaccine development. Recombinant vaccinia and MVA vectors expressing heterologous antigens have been developed as vaccine candidates against infectious diseases including HIV, Ebola, and tuberculosis, and MVA-based vaccines are currently licensed and deployed against mpox.. Vaccinia virus is also exploited as an oncolytic agent, with engineered, tumour-selective strains such as pexastimogene devacirepvec (JX-594) evaluated in clinical trials for their ability to preferentially replicate in and lyse malignant cells while stimulating anti-tumour immunity. In the laboratory, vaccinia virus underpins widely used transient gene expression systems, including the T7 RNA polymerase-vaccinia system, and continues to serve as a foundational model for studying poxvirus replication, innate immune evasion, and cytoplasmic DNA virus biology more broadly.

Vaccinia infection in immunocompetent individuals is typically self-limited, though adverse events including eczema vaccinatum, generalised vaccinia, and, rarely, progressive vaccinia can occur, particularly in people who are immunocompromised or have underlying skin conditions, which has driven continued interest in safer, replication-attenuated strains such as MVA for both prophylactic and vector applications.