**Background:** Gene therapy encompasses supplementation, correction, or modification of malfunctioning genes, with a broader definition including oligonucleotide-based silencing, RNA interference, immunotherapy, CAR T-cell therapy, CRISPR gene editing, and vaccine development. Viral vectors have played a central role due to superior gene delivery capacity compared to non-viral vectors, offering both short-term high-level expression (advantageous for cancer) and long-term expression (necessary for chronic diseases like hemophilia). The field was significantly set back in the 1990s by two tragic events: leukemia development in SCID-X1 patients due to retroviral integration into the LMO2 proto-oncogene, and the death of an 18-year-old patient from an adenovirus-based treatment for ornithine transcarbamylase deficiency. This review provides a comprehensive overview of viral vector systems and their utilization in preclinical studies and clinical trials as of 2023.
**Methods:** The review covers 13 viral vector types: adenovirus (Ad), adeno-associated virus (AAV), herpes simplex virus (HSV), retrovirus (RV), lentivirus (LV), alphavirus, flavivirus, measles virus (MV), rhabdovirus, Newcastle disease virus (NDV), poxvirus, picornavirus, reovirus, and polyoma virus. For each vector, the review describes genome type, packaging capacity, advantages, limitations, and engineering strategies. Applications are organized by disease area: cancer, cardiovascular diseases, metabolic diseases, hematological diseases, neurological disorders, muscular diseases, immunodeficiency, ophthalmologic diseases, lung diseases, and infectious disease vaccines.
**Key Results:** In cancer gene therapy, oncolytic viruses have demonstrated tumor eradication in animal models. For gliomas, SFV-IL-12 reduced RG2 glioma size by 87% in rats, and SFV VA7 completely eradicated 100% of small and 50% of large subcutaneous U87 tumors in mice. In melanoma, KUN-GM-CSF cured 67% of mice with B16-OVA melanomas. The oncolytic HSV T-VEC expressing GM-CSF was approved for advanced melanoma in the US, Europe, and Australia. For pancreatic cancer, HSV-HF10 in a phase I trial showed partial response in 3 patients, stable disease in 4, and progressive disease in 9. In ovarian cancer, MV-CEA in a phase I trial achieved stable disease in all 9 patients with median overall survival of 12.15 months (twice the expected time). For prostate cancer, VEE-PSCA provided long-term survival in 90% of TRAMP mice.
In cardiovascular disease, AAV1-SERCa2a in a phase IIa trial reduced the number of cardiovascular events and deaths. For hemophilia A, AAV-FVIII in a phase I/II study generated 8–60% of normal FVIII levels, and AAV5-hFVIII-SQ showed sustained clinically relevant benefits with decreased bleeding events. For hemophilia B, AAV8-FIX provided 1–6% of normal FIX levels for at least 3.2 years, and scAAV2-FIX showed stable FIX production for 7 years. AAV5-FVIII (BDD) received conditional marketing approval for severe hemophilia A from the EMA. In β-thalassemia, LentiGlobin BB305 allowed 12 patients with β0/β0 genotype to stop red blood cell transfusions in a phase I study, and phase III interim results confirmed sustained HbA^T87Q^ expression with transfusion independence for non-β0/β0 patients. For sickle cell disease, LentiGlobin BB305 in a phase I/II trial showed clinical remission in 2 of 3 patients.
In neurological disorders, AAV-hAAD in a phase I trial showed significant improvement in the Unified Parkinson's Disease Rating Scale sustained for at least 2 years. LV-ProSavin in a phase I/II follow-up study showed significant improvement in UPDRS score 4 years after treatment. For spinal muscular atrophy, AAV9-SMN (Zolgensma) demonstrated remarkable improvements in motor function and survival rates and was approved in the US, EU, and Canada. For Duchenne muscular dystrophy, AAV6-µDys in a phase I/II trial showed therapeutic levels of micro-dystrophin with 81% dystrophin-positive fibers and improvement in North Star Ambulatory Assessment scores in all patients. In immunodeficiency, SIN-γRV and SIN-LV vectors successfully treated 53 SCID-X1 patients without leukemia development, and more than 100 ADA-SCID patients have been treated with sustained ADA expression and high overall survival.
For COVID-19 vaccines, ChAdOx1 nCoV-19 showed 62–90% vaccine efficacy, rAd26-S/rAd5-S (Sputnik) showed 91.6% efficacy from interim results, and Ad26.COV2.S showed efficacy as a single dose. The VSV-based Ebola vaccine Ervebo was approved for vaccination against Ebola virus disease.
**Clinical Implications:** Viral vector-based gene therapy has transitioned from experimental to approved therapeutic modality across multiple disease areas. Approved viral-based drugs include Gendicine (adenovirus, p53) for head and neck cancer in China, T-VEC (oncolytic HSV, GM-CSF) for melanoma, Zolgensma (AAV9-SMN) for spinal muscular atrophy, Luxturna (AAV2-RPE65) for inherited retinal dystrophy, and Ervebo (VSV-ZEBOV) for Ebola. Multiple Ad-based COVID-19 vaccines received Emergency Use Authorization globally. The review emphasizes that vector choice must be tailored to indication: self-replicating RNA viruses for short-term high-level expression (cancer, vaccines), and Ad, AAV, HSV, RV, and LV vectors for long-term expression in inherited and chronic diseases. Key ongoing challenges include managing safety concerns related to chromosomal integration and oncolytic virus spread, difficulties translating findings from rodents to larger animals and humans, and the need for early-stage vector design to comply with clinical trial requirements.