Safety and immunogenicity of a synthetic multiantigen modified vaccinia virus Ankara-based COVID-19 vaccine (COH04S1): an open-label and randomised, phase 1 trial.

Background: COH04S1, a synthetic attenuated modified vaccinia virus Ankara vector co-expressing SARS-CoV-2 spike and nucleocapsid antigens, was tested for safety and immunogenicity in healthy adults. Methods: This combined open-label and randomised, phase 1 trial was done at the City of Hope Comprehensive Cancer Center (Duarte, CA, USA). We included participants aged 18-54 years with a negative SARS-CoV-2 antibody and PCR test, normal haematology and chemistry panels, a normal electrocardiogram and troponin concentration, negative pregnancy test if female, body-mass index of 30 kg/m2 or less, and no modified vaccinia virus Ankara or poxvirus vaccine in the past 12 months. In the open-label cohort, 1·0 × 107 plaque-forming units (PFU; low dose), 1·0 × 108 PFU (medium dose), and 2·5 × 108 PFU (high dose) of COH04S1 were administered by intramuscular injection on day 0 and 28 to sentinel participants using a queue-based statistical design to limit risk. In a randomised dose expansion cohort, additional participants were randomly assigned (3:3:1), using block size of seven, to receive two placebo vaccines (placebo group), one low-dose COH04S1 and one placebo vaccine (low-dose COH04S1 plus placebo group), or two low-dose COH04S1 vaccines (low-dose COH04S1 group). The primary outcome was safety and tolerability, with secondary objectives assessing vaccine-specific immunogenicity. The primary immunological outcome was a four times increase (seroconversion) from baseline in spike-specific or nucleocapsid-specific IgG titres within 28 days of the last injection, and seroconversion rates were compared with participants who received placebo using Fisher's exact test. Additional secondary outcomes included assessment of viral neutralisation and cellular responses. This trial is registered with ClinicalTrials.gov, NCT046339466. Findings: Between Dec 13, 2020, and May 24, 2021, 56 participants initiated vaccination. On day 0 and 28, 17 participants received low-dose COH04S1, eight received medium-dose COH04S1, nine received high-dose COH04S1, five received placebo, 13 received low-dose COH04S1 followed by placebo, and four discontinued early. Grade 3 fever was observed in one participant who received low-dose COH04S1 and placebo, and grade 2 anxiety or fatigue was seen in one participant who received medium-dose COH04S1. No severe adverse events were reported. Seroconversion was observed in all 34 participants for spike protein and 32 (94%) for nucleocapsid protein (p<0·0001 vs placebo for each comparison). Four times or more increase in SARS-CoV-2 neutralising antibodies within 56 days was measured in nine of 17 participants in the low-dose COH04S1 group, all eight participants in the medium-dose COH04S1 group, and eight of nine participants in the high-dose COH04S1 group (p=0·0035 combined dose levels vs placebo). Post-prime and post-boost four times increase in spike-specific or nucleocapsid-specific T cells secreting interferon-γ was measured in 48 (98%; 95% CI 89-100) of 49 participants who received at least one dose of COH04S1 and provided a sample for immunological analysis. Interpretation: COH04S1 was well tolerated and induced spike-specific and nucleocapsid-specific antibody and T-cell responses. Future evaluation of this COVID-19 vaccine candidate as a primary or boost vaccination is warranted. Funding: The Carol Moss Foundation and City of Hope Integrated Drug Development Venture programme.

Exploring the role of oncolytic viruses in hepatobiliary cancers.

The standard of care for early hepatobiliary cancers (HBC) includes surgical resection. Liver transplantations or locoregional therapies are beneficial in early hepatocellular carcinoma (HCC) under certain circumstances. Systemic treatments have some benefit in advanced HBC, though long-term prognosis remains poor. We evaluated the role of oncolytic viruses in the treatment of HBCs through a systematic literature review. The recombinant vaccinia virus JX-594 improved median survival in patients with local/metastatic HCC more strongly at high dose than at low dose (14.1 vs 6.7 months; p = 0.08) in a Phase II study. A Phase III study with JX-594 and sorafenib in advanced HCC is ongoing. No survival benefit in HCC was seen with two other recombinant adenoviruses (Ad-TK and DL1520). Several preclinical trials using oncolytic viruses in HBC showed promising results, warranting clinical studies.

Retinaldehyde dehydrogenase 2 as a molecular adjuvant for enhancement of mucosal immunity during DNA vaccination.

In order for vaccines to induce efficacious immune responses against mucosally transmitted pathogens, such as HIV-1, activated lymphocytes must efficiently migrate to and enter targeted mucosal sites. We have previously shown that all-trans retinoic acid (ATRA) can be used as a vaccine adjuvant to enhance mucosal CD8+ T cell responses during vaccination and improve protection against mucosal viral challenge. However, the ATRA formulation is incompatible with most recombinant vaccines, and the teratogenic potential of ATRA at high doses limits its usage in many clinical settings. We hypothesized that increasing in vivo production of retinoic acid (RA) during vaccination with a DNA vector expressing retinaldehyde dehydrogenase 2 (RALDH2), the rate-limiting enzyme in RA biosynthesis, could similarly provide enhanced programming of mucosal homing to T cell responses while avoiding teratogenic effects. Administration of a RALDH2- expressing plasmid during immunization with a HIVgag DNA vaccine resulted in increased systemic and mucosal CD8+ T cell numbers with an increase in both effector and central memory T cells. Moreover, mice that received RALDH2 plasmid during DNA vaccination were more resistant to intravaginal challenge with a recombinant vaccinia virus expressing the same HIVgag antigen (VACVgag). Thus, RALDH2 can be used as an alternative adjuvant to ATRA during DNA vaccination leading to an increase in both systemic and mucosal T cell immunity and better protection from viral infection at mucosal sites.

Binding of undamaged double stranded DNA to vaccinia virus uracil-DNA Glycosylase.

BACKGROUND: Uracil-DNA glycosylases are evolutionarily conserved DNA repair enzymes. However, vaccinia virus uracil-DNA glycosylase (known as D4), also serves as an intrinsic and essential component of the processive DNA polymerase complex during DNA replication. In this complex D4 binds to a unique poxvirus specific protein A20 which tethers it to the DNA polymerase. At the replication fork the DNA scanning and repair function of D4 is coupled with DNA replication. So far, DNA-binding to D4 has not been structurally characterized. RESULTS: This manuscript describes the first structure of a DNA-complex of a uracil-DNA glycosylase from the poxvirus family. This also represents the first structure of a uracil DNA glycosylase in complex with an undamaged DNA. In the asymmetric unit two D4 subunits bind simultaneously to complementary strands of the DNA double helix. Each D4 subunit interacts mainly with the central region of one strand. DNA binds to the opposite side of the A20-binding surface on D4. Comparison of the present structure with the structure of uracil-containing DNA-bound human uracil-DNA glycosylase suggests that for DNA binding and uracil removal D4 employs a unique set of residues and motifs that are highly conserved within the poxvirus family but different in other organisms. CONCLUSION: The first structure of D4 bound to a truly non-specific undamaged double-stranded DNA suggests that initial binding of DNA may involve multiple non-specific interactions between the protein and the phosphate backbone.

Protective Efficacy of Recombinant Modified Vaccinia Virus Ankara Delivering Middle East Respiratory Syndrome Coronavirus Spike Glycoprotein.

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe respiratory disease in humans. We tested a recombinant modified vaccinia virus Ankara (MVA) vaccine expressing full-length MERS-CoV spike (S) glycoprotein by immunizing BALB/c mice with either intramuscular or subcutaneous regimens. In all cases, MVA-MERS-S induced MERS-CoV-specific CD8(+) T cells and virus-neutralizing antibodies. Vaccinated mice were protected against MERS-CoV challenge infection after transduction with the human dipeptidyl peptidase 4 receptor. This MERS-CoV infection model demonstrates the safety and efficacy of the candidate vaccine.

Progress in clinical oncolytic virus-based therapy for hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) carries a dismal prognosis, with advanced disease being resistant to both radiotherapy and conventional cytotoxic drugs, whilst anti-angiogenic drugs are marginally efficacious. Oncolytic viruses (OVs) offer the promise of selective cancer therapy through direct and immune-mediated mechanisms. The premise of OVs lies in their preferential genomic replication, protein expression and productive infection of malignant cells. Numerous OVs are being tested in preclinical models of HCC, with good evidence of direct and immune-mediated anti-tumour efficacy. Efforts to enhance the performance of these agents have concentrated on engineering OV cellular specificity, immune evasion, enhancing anti-tumour potency and improving delivery. The lead agent in HCC clinical trials, JX-594, a recombinant Wyeth strain vaccinia virus, has demonstrated evidence for significant benefit and earned orphan drug status. Thus, JX-594 appears to be transcending the barrier between novel laboratory science and credible clinical therapy. Relatively few other OVs have entered clinical testing, a hurdle that must be overcome if significant progress is to be made in this field. This review summarizes the preclinical and clinical experience of OV therapy in the difficult-to-treat area of HCC.

Preclinical testing of virotherapeutics for primary and secondary tumors of the liver.

Virotherapy offers a new treatment strategy using oncolytic viruses as self-replicating, tumor-specific agents, which destroy tumors during their natural lytic replication process. To study potential oncolytic viruses, cell culture experiments give basic information about the lytic potential of a virus, measured as cell lysis or decreased viability. For further analysis, animal models are usually employed. As these in vivo experiments are often performed in immunocompromised animals, results have to be interpreted with caution. Therefore, to obtain deeper information of the oncolytic action of specific viruses in a patient's individual context we established a test platform based on human primary tissue slices. In this three-dimensional model, we observed a preferential tumor infection and the penetration of oncolytic measles vaccine virus into deeper cell layers of tumor tissues, which is an essential feature of an effective oncolytic virus.

Targeted and armed oncolytic poxviruses for cancer: the lead example of JX-594.

Oncolytic viruses (OVs) are designed to replicate in, and subsequently lyse cancer cells. Numerous oncolytic virus platforms are currently in development. Here we review preclinical and clinical experience with JX-594, the lead candidate from the targeted and armed oncolytic poxvirus class. JX-594 is derived from a vaccinia vaccine strain that has been engineered for 1) enhanced cancer targeting and 2) has been "armed" with the therapeutic transgene granulocytemacrophage colony stimulating factor (GM-CSF) to stimulate anti-tumoral immunity. Poxviruses have many ideal features for use as oncolytic agents. The development of oncolytic vaccinia viruses is supported by a large safety database accumulated in the smallpox eradication program. In addition, poxviruses have evolved unique capabilities for systemic spread through the blood that can be harnessed for the treatment of metastatic disease. JX-594 demonstrates a high degree of cancer selectivity and systemic efficacy by multiple mechanisms-of-action (MOAs) in preclinical testing. Data from Phase 1 and 2 clinical trials has confirmed that these features result in potent and systemic efficacy in patients with treatment refractory metastatic cancers.

Retinoic acid as a vaccine adjuvant enhances CD8+ T cell response and mucosal protection from viral challenge.

Vaccine-induced memory T cells localized at mucosal sites can provide rapid protection from viral infection. All-trans-retinoic acid (ATRA) has been shown to act physiologically to induce the expression of gut-homing receptors on lymphocytes. We tested whether the administration of exogenous ATRA during a systemic vaccination of mice could enhance the generation of mucosal CD8(+) T cell immunity, which might represent a strategy for establishing better protection from viral infection via mucosal routes. ATRA induced the expression of CCR9 and α4ß7 on both mouse and human CD8(+) T cells activated in vitro. The administration of ATRA to mice during in vivo priming with a replication-defective recombinant adenovirus vector expressing the lymphocytic choriomeningitis virus glycoprotein (LCMVgp) (Ad5gp) increased numbers of both effector and memory T cells in intestinal mucosal tissues and showed higher frequencies of systemic central memory-like T cells that exhibited enhanced proliferation during boosting immunization with recombinant modified vaccinia virus Ankara expressing LCMVgp (MVAgp). Mice that received ATRA during Ad5gp vaccination were more resistant to intravaginal challenge by recombinant vaccinia virus expressing LCMVgp (VVgp), reflecting in part stronger T cell recall responses in situ. Thus, ATRA appears to be useful as an adjuvant during vaccination to increase memory T cell responses and protection from viral infection at mucosal sites and may facilitate the development of more effective vaccines against mucosally transmitted pathogens such as HIV.

Sequential therapy with JX-594, a targeted oncolytic poxvirus, followed by sorafenib in hepatocellular carcinoma: preclinical and clinical demonstration of combination efficacy.

JX-594 is a targeted and granulocyte-macrophage colony stimulating factor (GM-CSF) expressing oncolytic poxvirus designed to selectively replicate in and destroy cancer cells through viral oncolysis and tumor-specific immunity. In a phase 1 trial, JX-594 injection into hepatocellular carcinoma (HCC) was well-tolerated and associated with viral replication, decreased tumor perfusion, and tumor necrosis. We hypothesized that JX-594 and sorafenib, a small molecule inhibitor of B-raf and vascular endothelial growth factor receptor (VEGFR) approved for HCC, would have clinical benefit in combination given their demonstrated efficacy in HCC patients and their complementary mechanisms-of-action. HCC cell lines were uniformly sensitive to JX-594. Anti-raf kinase effects of concurrent sorafenib inhibited JX-594 replication in vitro, whereas sequential therapy was superior to either agent alone in murine tumor models. We therefore explored pilot safety and efficacy of JX-594 followed by sorafenib in three HCC patients. In all three patients, sequential treatment was (i) well-tolerated, (ii) associated with significantly decreased tumor perfusion, and (iii) associated with objective tumor responses (Choi criteria; up to 100% necrosis). HCC historical control patients on sorafenib alone at the same institutions had no objective tumor responses (0 of 15). Treatment of HCC with JX-594 followed by sorafenib has antitumoral activity, and JX-594 may sensitize tumors to subsequent therapy with VEGF/VEGFR inhibitors.

Isolation of virus-cell fusion inhibitory components from the stem bark of Styrax japonica S. et Z.

Five compounds, styraxjaponoside A (1), matairesinoside (2), egonol glucoside (3), dihydrodehydrodiconiferyl alcohol 9'-O-glucoside (4), and styraxjaponoside B (5) were isolated from the stem bark of Styrax japonica. Among them, compounds 1 and 5 showed significantly high virus-cell fusion inhibitory activity. In addition, compound 5 exhibited almost equivalent virus-cell fusion inhibitory activity to that of dextran sulfate, which is used as a positive control.

Identification of polymerase and processivity inhibitors of vaccinia DNA synthesis using a stepwise screening approach.

Nearly all DNA polymerases require processivity factors to ensure continuous incorporation of nucleotides. Processivity factors are specific for their cognate DNA polymerases. For this reason, the vaccinia DNA polymerase (E9) and the proteins associated with processivity (A20 and D4) are excellent therapeutic targets. In this study, we show the utility of stepwise rapid plate assays that (i) screen for compounds that block vaccinia DNA synthesis, (ii) eliminate trivial inhibitors, e.g. DNA intercalators, and (iii) distinguish whether inhibitors are specific for blocking DNA polymerase activity or processivity. The sequential plate screening of 2222 compounds from the NCI Diversity Set library yielded a DNA polymerase inhibitor (NSC 55636) and a processivity inhibitor (NSC 123526) that were capable of reducing vaccinia viral plaques with minimal cellular cytotoxicity. These compounds are predicted to block cellular infection by the smallpox virus, variola, based on the very high sequence identity between A20, D4 and E9 of vaccinia and the corresponding proteins of variola.

Vaccinia viruses with mutations in the E3L gene as potential replication-competent, attenuated vaccines: intra-nasal vaccination.

Vaccinia virus (VACV) has been used as the vaccine to protect against smallpox, and recombinant VACVs have been used to develop vaccine candidates against numerous cancers and infectious diseases. Although relatively safe for use in humans, the strains of VACV that were used as smallpox vaccines led to several complications including, progressive infection in immune compromised individuals, eczema vaccination in individuals with a history of atopic dermatitis, and encephalitis and perimyocarditis in apparently healthy individuals. The work described in this paper focuses on attenuated strains of VACV that may have the potential for use as vaccine vectors with reduced pathogenicity. We have generated several VACV mutants in a WR background with specific mutations in the E3L gene that were at least a 1000-fold less pathogenic compared to wtVACV upon intra-nasal infection of mice. Many of these mutant viruses replicated to high titers in the nasal mucosa of mice following intra-nasal administration. Despite replication to high titers in the nose, there was little spread to other organs in infected animals. Intra-nasal vaccination with doses as low as 100-1000 pfu (plaque forming units) of these replicating VACV constructs were sufficient to protect the host from challenge with large doses of wtVACV. Similar constructs in a Copenhagen and a NYCBH background were highly attenuated, yet effective as vaccines in the mouse model. These recombinant VACV constructs may be promising vector candidates for use in vaccination strategies against smallpox and other pathogens.

Identification of novel antipoxviral agents: mitoxantrone inhibits vaccinia virus replication by blocking virion assembly.

The bioterror threat of a smallpox outbreak in an unvaccinated population has mobilized efforts to develop new antipoxviral agents. By screening a library of known drugs, we identified 13 compounds that inhibited vaccinia virus replication at noncytotoxic doses. The anticancer drug mitoxantrone is unique among the inhibitors identified in that it has no apparent impact on viral gene expression. Rather, it blocks processing of viral structural proteins and assembly of mature progeny virions. The isolation of mitoxantrone-resistant vaccinia strains underscores that a viral protein is the likely target of the drug. Whole-genome sequencing of mitoxantrone-resistant viruses pinpointed missense mutations in the N-terminal domain of vaccinia DNA ligase. Despite its favorable activity in cell culture, mitoxantrone administered intraperitoneally at the maximum tolerated dose failed to protect mice against a lethal intranasal infection with vaccinia virus.

Discovery of small molecule inhibitors of ubiquitin-like poxvirus proteinase I7L using homology modeling and covalent docking approaches.

Essential for viral replication and highly conserved among poxviridae, the vaccinia virus I7L ubiquitin-like proteinase (ULP) is an attractive target for development of smallpox antiviral drugs. At the same time, the I7L proteinase exemplifies several interesting challenges from the rational drug design perspective. In the absence of a published I7L X-ray structure, we have built a detailed 3D model of the I7L ligand binding site (S2-S2' pocket) based on exceptionally high structural conservation of this site in proteases of the ULP family. The accuracy and limitations of this model were assessed through comparative analysis of available X-ray structures of ULPs, as well as energy based conformational modeling. The 3D model of the I7L ligand binding site was used to perform covalent docking and VLS of a comprehensive library of about 230,000 available ketone and aldehyde compounds. Out of 456 predicted ligands, 97 inhibitors of I7L proteinase activity were confirmed in biochemical assays ( approximately 20% overall hit rate). These experimental results both validate our I7L ligand binding model and provide initial leads for rational optimization of poxvirus I7L proteinase inhibitors. Thus, fragments predicted to bind in the prime portion of the active site can be combined with fragments on non-prime side to yield compounds with improved activity and specificity.

Rescue system for measles virus from cloned cDNA driven by vaccinia virus Lister vaccine strain.

A rescue system for measles virus from cloned cDNA was established using CHO/hSLAM cells (Chinese hamster ovary cells expressing a measles virus receptor, signaling lymphocyte activation molecule), LO-T7-1 virus (the Lister vaccine strain of vaccinia virus expressing the T7 RNA polymerase under the control of the early/late p7.5 promoter), and caspase inhibitor. LO-T7-1 drove efficiently the T7 promoter in CHO/hSLAM cells. Rescue efficiency with LO-T7-1 was not as high as that with the vTF7-3 strain based on a neurovirulent vaccinia virus, but was increased by using a caspase inhibitor to block apoptosis of CHO/hSLAM cells induced by LO-T7-1. These modifications resulted in a measles virus rescue efficiency that was even higher than that of previous systems. This safer and more efficient system will facilitate further the genetic manipulation of measles virus in basic research and virus vector development.

The use of oncolytic vaccinia viruses in the treatment of cancer: a new role for an old ally?

The use of genetically engineered, tumor-targeting viruses as oncolytic agents has recently emerged as a promising new area for the development of novel cancer therapies. The first viruses to enter the clinic, such as ONYX-015 (an oncolytic adenovirus), provided evidence both for the safety and for the anti-tumor potential of this approach. The results of these early trials have also allowed investigators to examine the limitations of these viruses and to develop potentially far more effective approaches. In this review the development of such next generation viruses, in particular the potential use of strains of vaccinia virus, will be discussed. Vaccinia has an enormous history of use in humans and possesses many of the features felt to be beneficial for the creation of a successful virotherapy agent. It causes no known disease in humans, yet is capable of infecting almost all cell types with a subsequent rapid and lytic infection, which subsequently induces a vigorous local CTL immune response at the site of infection. Vaccinia also displays natural tumor tropism, and several approaches have been used to further limit viral replication to tumor cells and to optimize the immune response induced at the site of the tumor. Finally, the large cloning capacity of vaccinia allows for the addition of multiple foreign genes into the viral genome. This has been exploited to increase the bystander effect of the virus by immune modulation or by expression of pro-drug converting enzymes as well as to incorporate safety controls and reporters for in vivo molecular imaging. Initial clinical trials with these viruses further highlights their potential as the next generation of oncolytic agents and as highly effective future cancer therapies.

Targeting vaccinia to solid tumors with local hyperthermia.

We have previously demonstrated that mutant vaccinia viruses target tumors in vivo after systemic delivery, and they have potential as vectors for tumor-directed gene therapy. We hypothesized that hyperthermia may augment vaccinia delivery to tumors after systemic injection, as hyperthermia increases the permeability of the endothelial vasculature to nanoparticles. In our in vitro experiments, we have shown that hyperthermia does not alter tumor cells' susceptibility to the intrinsic cytopathogenicity of the vaccinia virus compared with normothermic controls. Hyperthermia also does not change the viral infectivity or the level of viral marker gene expression when compared with normothermia. In an in vitro model of endothelial cell monolayer permeability, we have demonstrated that hyperthermia increases the permeability of the monolayer to vaccinia virus and that this phenomenon is completely reversible. In vivo we have demonstrated that the tumors that were treated with systemic vaccinia under conditions of hyperthermia (41.5 degrees C for 30 min) had significantly higher levels of vaccinia marker gene activity (>100-fold) than those treated under normothermic conditions (p < 0.05) and that this effect was specific to tumor. We also demonstrated that mice with 1 cm subcutaneous tumors treated with a systemically delivered, conditionally replicating vaccinia under conditions of hyperthermia had complete tumor regression in 50% and significantly improved antitumor response, compared with normothermic viral-treated controls (mean tumor volume of 110 mm(3) vs 3169 mm(3), 13 days after treatment) and compared with hyperthermic, nonvirally treated control animals (p < 0.0001). Regional hyperthermia improves vaccinia targeting to tumors, and thereby enhances the antitumor response.

Biodistribution and persistence of an MVA-vectored candidate HIV vaccine in SIV-infected rhesus macaques and SCID mice.

Recombinant modified vaccinia virus Ankara (MVA) is together with a few other attenuated viral vectors on the forefront of human immunodeficiency virus type 1 (HIV-1) vaccine development. As such, MVA-vectored vaccines are likely to be administered into immunocompromized individuals. Here, we demonstrated in a good laboratory practice study safety and biological clearance of candidate HIV-1 vaccine MVA.HIVA in simian immunodeficiency virus (SIV)-infected rhesus macaques and mice with a severe combined immunodeficiency (SCID) following an intradermal vaccine administration. In SIV-infected macaques, MVA.HIVA DNA was undetectable by nested PCR 6 weeks after dosing. In SCID mice, the MVA.HIVA vaccine was well tolerated and a positive PCR signal was only observed at the site of injection 49 days after dosing in four out of six mice, but even these sites were negative by day 81 post-injection. Therefore, the MVA.HIVA vaccine is considered safe for application in phase I clinical trials in HIV-1-infected human subjects. These results also contribute to the confidence of using MVA as a smallpox vaccine.

Future directions for the field of oncolytic virotherapy: a perspective on the use of vaccinia virus.

Oncolytic virotherapy is an emerging biotherapeutic platform based on genetic engineering of viruses capable of selectively infecting and replicating within cancer cells. Such viruses have been found to be both safe and to produce antitumour effects in a number of Phase I and II clinical trials. Early work in this field has been pioneered with strains of adenovirus which, although well suited to gene therapy approaches, have displayed certain limitations in their ability to directly destroy and spread through tumour tissues, particularly after systemic administration. Investigators have subsequently been examining the feasibility of using a variety of different viruses as oncolytic agents. Vaccinia virus is perhaps the most widely administered and successful medical product in history; it displays many of the qualities thought necessary for an effective antitumour agent and is particularly well characterised in people due to its role in the eradication of smallpox. Vaccinia has a short life cycle and rapid spread, strong lytic ability, inherent systemic tumour targeting, a large cloning capacity and well-defined molecular biology. In addition, the virus produces no known disease in humans, has been delivered safely to millions of people and has already demonstrated antitumoural efficacy in trials with vaccine strains. These qualities, along with strategies for further improving the safety and antitumour effectiveness of vaccinia, will be discussed in relation to the broad spectrum of clinical experience already achieved with this virus in cancer therapy.