Strip a virus of its pathogenic genes, replace them with therapeutic ones, and you have a delivery system evolution spent millions of years perfecting. A single injection that rewrites cellular instructions to correct a genetic disease, target cancer, or prime immunity against infection; this is the promise of viral vector technology. Several approved therapies now demonstrate this is more than theoretical.

Viral vectors are sub-microscopic vehicles that deliver therapeutic genes directly into cells. The approach involves taking a virus, removing its pathogenic genetic components, and replacing them with therapeutic DNA or RNA. Once administered, these modified viruses target specific cell types where they can correct a genetic defect, restore production of a missing protein, or train the immune system to recognise a pathogen.

The principle resembles correcting faulty code in a program. If a gene is defective or absent, the viral vector delivers a functional copy to restore normal cellular function. The value of viral vectors lies in their precision — different virus types can target specific tissues, including liver, brain, or immune cells, depending on the therapeutic goal.

A Brief History of Viral Vectors

The concept of harnessing viruses therapeutically emerged in the 1970s. Viruses have evolved over millions of years to enter cells and hijack their genetic machinery for replication. By the 1980s, researchers began experimenting with retroviruses (a family that integrates genetic material into host DNA). By removing harmful viral genes and inserting therapeutic sequences, they created the first viral vectors.

Early clinical trials in the 1990s showed promise but encountered significant obstacles. A critical setback occurred in 1999 when 18-year-old Jesse Gelsinger died during a gene therapy trial at the University of Pennsylvania using an adenoviral vector. Gelsinger, who had ornithine transcarbamylase deficiency, experienced a severe systemic inflammatory response to the vector and died four days after receiving the injection. This was the first death directly attributable to a gene delivery vehicle, and it halted gene therapy trials across the United States.

Over the following two decades, researchers made substantial advances in vector design, immune response management, and delivery methods. By the 2010s, the field had recovered. In December 2017, the US Food and Drug Administration (FDA) approved Luxturna (voretigene neparvovec-rzyl), the first gene therapy for an inherited genetic disease in the United States. Luxturna uses an adeno-associated viral (AAV) vector to deliver a functional RPE65 gene to retinal cells in patients with Leber congenital amaurosis. In clinical trials, patients who had been legally blind regained the ability to perceive shapes and light. Some could read. The treatment involves a single subretinal injection per eye, with benefits persisting for years.

This approval demonstrated that viral vectors could deliver transformative treatments safely. Since then, applications have expanded across genetic disorders, cancer, and infectious diseases.

How Viral Vectors Work

The most commonly used vectors derive from adenoviruses, adeno-associated viruses (AAVs), lentiviruses, and retroviruses. Each has distinct properties that determine its therapeutic application.

Adeno-associated viruses (AAVs): AAVs are small, non-pathogenic viruses that have become the preferred platform for gene therapy. They efficiently enter non-dividing cells, including those in the eye, muscle, and liver, and generally trigger limited immune responses. Importantly, AAV vectors remain separate from the host’s chromosomes rather than inserting into them — reducing the risk of disrupting other genes. Luxturna (for inherited retinal dystrophy), Zolgensma (for spinal muscular atrophy), and Hemgenix (for haemophilia B) all use AAV vectors.

Lentiviruses: Derived from HIV, lentiviral vectors can integrate into the genome of both dividing and non-dividing cells. The genetic changes are permanent, making lentiviruses ideal for therapies where cells are modified outside the body and reinfused. CAR-T cancer treatments use this approach.

Adenoviruses: These vectors produce transient gene expression without integrating into host DNA, making them suitable for applications requiring short-term effects. Adenoviral vectors formed the basis of several COVID-19 vaccines and have been used in Ebola vaccination programmes, where they generate robust antibody and T cell responses.

Other retroviruses: Gamma-retroviral vectors, derived from mouse leukaemia viruses, were used in early gene therapy trials for severe combined immunodeficiency (SCID). However, their tendency to integrate near oncogenes caused leukaemia in some patients, leading to reduced use in favour of safer alternatives.

How Are Viral Vectors Produced?

Viral vector manufacturing involves culturing host cells, transfecting them with viral and therapeutic gene components, harvesting the modified viruses, and purifying them for clinical use. The process varies depending on virus type, application, and scale.

Vector design: Selection of an appropriate virus type depends on the target tissue, genetic payload capacity, and safety profile. Pathogenic genes (particularly those required for replication) are removed while retaining essential elements for packaging and gene delivery.

Transgene cloning: The therapeutic gene is inserted into the viral genome using standard molecular cloning techniques, then incorporated into a plasmid (a circular DNA molecule that can be transferred between cells).

Vector production: Producer cells (commonly HEK293, a widely used laboratory cell line) are transfected with plasmids carrying the therapeutic gene alongside those encoding viral structural components. Over 48–72 hours, cells produce viral proteins that self-assemble into particles containing the therapeutic transgene.

Harvesting and purification: Depending on the virus type, vectors are collected either by breaking open the producer cells or from the surrounding culture fluid. Purification involves multiple steps (filtration, chromatography, and ultracentrifugation) to remove cellular debris and concentrate the final product.

Current Therapeutic Applications

Viral vectors are being applied across three major areas: gene therapy for genetic disorders, cancer immunotherapy, and vaccine development.

Gene Therapy for Genetic Disorders

Gene therapy aims to correct or replace defective genes causing disease. Viral vectors serve as the delivery mechanism, transporting functional gene copies into target cells.

Spinal muscular atrophy: In May 2019, the FDA approved Zolgensma (onasemnogene abeparvovec-xioi) for children under two years with spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. SMA results from mutations in the SMN1 gene, leading to motor neuron loss and progressive muscle weakness. Zolgensma uses an AAV9 vector to deliver a functional SMN1 gene to motor neurons via a single intravenous infusion. In clinical trials, infants treated with Zolgensma achieved motor milestones (including sitting and, in some cases, walking) never observed in the natural history of the disease. Follow-up data demonstrate that motor gains have persisted for years after treatment.

Haemophilia: In November 2022, the FDA approved Hemgenix (etranacogene dezaparvovec-drlb), the first gene therapy for haemophilia B. Hemgenix uses an AAV5 vector to deliver a functional factor IX gene to liver cells, where clotting factors are normally produced. In the pivotal phase 3 trial, 96% of participants (52 of 54) discontinued factor IX prophylaxis, with the annualized bleeding rate reduced by 54% compared to baseline. Factor IX activity levels remained elevated at 18-month follow-up, demonstrating durable therapeutic effect.

Cancer Immunotherapy

Viral vectors have enabled CAR-T cell therapy — a treatment that reprograms a patient’s own immune cells to recognise and attack cancer.

The process works in stages. First, T cells (a type of white blood cell) are collected from the patient’s blood. In the laboratory, a lentiviral vector inserts a new gene that encodes a chimeric antigen receptor (CAR) (a synthetic protein designed to lock onto markers found on cancer cells). The modified cells are multiplied, then infused back into the patient, where they seek out and destroy the tumour.

In August 2017, the FDA approved Kymriah (tisagenlecleucel), the first CAR-T therapy and the first gene therapy product approved in the United States. Kymriah treats B-cell acute lymphoblastic leukaemia (ALL) that has returned or stopped responding to other treatments, in patients up to 25 years of age. The lentiviral vector equips T cells with a CAR targeting CD19, a protein found on the surface of these leukaemia cells. In the pivotal ELIANA trial, 82% of patients achieved remission, with approximately half maintaining remission at 36 months and 63% overall survival. Subsequent approvals extended Kymriah to adult diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma.

Research is ongoing to extend CAR-T approaches to solid tumours, including lung and pancreatic cancer, though these present additional hurdles — solid tumours suppress immune activity in their surroundings and lack uniform surface markers for T cells to target.

Oncolytic viral vectors represent another application. Imlygic (talimogene laherparepvec), approved in 2015 for melanoma, uses a modified herpes simplex virus that selectively infects and lyses tumour cells while stimulating anti-tumour immune responses.

Vaccine Development

Viral vectors can deliver genetic instructions for antigens (molecules that trigger immune responses), enabling the host’s cells to produce the target protein and prime the immune system.

Adenoviral vector vaccines played a significant role in the COVID-19 pandemic response. The Oxford-AstraZeneca vaccine uses a modified chimpanzee adenovirus to deliver the SARS-CoV-2 spike protein gene; the Janssen/Johnson & Johnson vaccine uses a human adenovirus. These vaccines demonstrated efficacy rates of 60–80% against symptomatic COVID-19 infection and contributed substantially to the global vaccination effort.

Adenoviral vectors were chosen partly because they can be manufactured at scale and stored at refrigerator temperatures, unlike mRNA vaccines requiring ultra-cold storage. However, rare thrombotic events associated with some adenoviral COVID-19 vaccines led to modified recommendations in several countries.

Beyond COVID-19, adenoviral vectors are being evaluated for vaccines against Ebola, Zika, and HIV. Viral vector-based Ebola vaccines were deployed during West African and Democratic Republic of Congo outbreaks. Phase 2 trials for an adenoviral HIV vaccine have shown promising immune responses, though an effective preventive vaccine remains elusive.

Challenges and Limitations

Despite progress, viral vector therapies face significant challenges

Manufacturing complexity and cost: Production of clinical-grade viral vectors is technically demanding and expensive. Zolgensma costs approximately £1.7 million per dose, and Hemgenix was priced at $3.5 million, limiting accessibility. Manufacturing requires specialised facilities and extensive quality control testing.

Immunogenicity: Pre-existing immunity to common viral vectors (particularly adenoviruses and some AAV serotypes) can neutralise vectors before they reach target cells. Additionally, immune responses to the vector or its payload can limit treatment efficacy and prevent re-dosing. Researchers are engineering vector shells that the immune system fails to recognise.

Insertional mutagenesis: Integrating vectors (lentiviruses, gamma-retroviruses) carry a risk of disrupting tumour suppressor genes or activating oncogenes. This risk became evident when some SCID patients treated with gamma-retroviral gene therapy developed leukaemia. Newer vector designs include safety features that limit where and how actively the inserted gene behaves.

Limited payload capacity: AAV vectors can carry roughly 4,700 DNA base pairs — enough for small to medium genes, but insufficient for larger therapeutic sequences. Strategies to address this include splitting the gene across two vectors or using shortened gene variants.

Safety: In 2025, several deaths were linked to AAV vector–associated hepatotoxicity, prompting the suspension of multiple programs and a renewed reassessment of the suitability of these vectors for specific diseases and patient populations. High dosing requirements, liver toxicity, and manufacturing variability have emerged as critical concerns in the field that demand urgent attention. Efforts to de-risk AAV vectors through targeted delivery strategies that avoid the liver have thus far proven unsuccessful, even in cases where in vivo studies in non-human primates had suggested a more favourable outcome.

Durability and long-term safety: While some gene therapies provide durable benefit, transgene expression can wane over time, particularly in dividing tissues. Long-term safety monitoring remains essential, as the full consequences of gene transfer may take years to manifest.

Complex diseases: Current gene therapies work best for single-gene conditions. Diseases involving multiple genes or complex pathophysiology, such as Alzheimer’s disease or type 2 diabetes, present greater challenges that existing vectors cannot fully address.

Future Directions

Several developments may expand the therapeutic scope of viral vectors:

Gene editing integration: Combining viral vectors with CRISPR and other gene-editing tools enables precise, targeted genomic corrections. Clinical trials using AAV vectors to deliver CRISPR components for sickle cell disease and beta-thalassaemia have shown promising results, with some patients achieving near-normal haemoglobin levels.

Neurological applications: AAV vectors are being investigated for neurodegenerative conditions including Parkinson’s and Alzheimer’s disease, with some improvement in motor symptoms reported.

Novel delivery approaches: Hybrid vectors combining viral and non-viral elements, along with redesigned viral shells that home to specific tissues more efficiently, may improve targeting while reducing immune reactions. These approaches could enable access to challenging tissues, including the brain and heart.

Manufacturing advances: Cell-free production systems, suspension bioreactor processes, and improved purification methods aim to reduce manufacturing costs. Industry projections suggest gene therapies could become more accessible as production efficiency improves.

Preventive applications: Exploratory research is investigating whether viral vectors could prevent disease before symptoms develop. Early studies are evaluating prophylactic gene therapies for hereditary cancers, delivering genes that block cancer-causing mutations.

The Path Ahead

Viral vector technology has progressed from early setbacks to regulatory approval of multiple transformative therapies. From restoring vision with Luxturna to treating SMA with Zolgensma to enabling CAR-T immunotherapy with Kymriah, these technologies are addressing previously intractable conditions. The core mechanisms (efficient cell entry, tissue-specific targeting, and durable gene expression) have proven clinically tractable, though challenges in manufacturing, immunogenicity, liver toxicity, and long-term safety remain. Whether a decade of sustained research will now translate into the broad therapeutic impact long anticipated remains uncertain; however, the applicability of gene therapy and the course corrections required for the field’s advancement are likely to become clearer in the next few years.

Blog by Paul Griffin

Supported by Reckon Better