Viral vectors and viral particles are powerful tools for delivering recombinant genetic material into cells, both in vitro and in vivo.
These systems enable reliable transduction of custom genes for either stable or transient protein expression.
In addition to protein-coding cDNA clones or open reading frames (ORFs), viral delivery can be used to introduce RNA molecules such as shRNA and miRNA for RNA interference (RNAi) and gene silencing applications.
Components for gene editing technologies, including CRISPR/Cas systems, are also widely available in viral formats.
Compared to traditional plasmid-based transfection, viral delivery offers several advantages and is a promising platform for gene therapy.
Here, we provide a brief guide to different viral vectors and highlight their applications in research and therapeutics.
Viral versus non-viral vectors
Non-viral delivery methods rely on a carrier for the transgene and a means of crossing the target cell membrane.
Common techniques include calcium phosphate precipitation, lipid- or liposome-based transfection (lipofection), electroporation, and microinjection.
These approaches are widely used due to their flexibility, relatively low cost, and absence of the biosafety concerns associated with viral systems.
However, non-viral methods typically have lower delivery efficiency, limited cell-type specificity, and shorter durations of gene expression, which can limit their usefulness in certain applications.
Viral vectors, in contrast, offer several advantages that make them especially valuable in therapeutic and gene therapy contexts.
They can support both long-term transgene expression—ideal for treating chronic conditions—and short-term, high-level expression, which is advantageous for cancer therapies.
Many viral systems also allow a degree of control over regulation or termination of gene expression and tissue-specific targeting.
This precision and adaptability have driven their growing use in both in vivo and ex vivo therapeutic strategies.
In in vivo applications, viral vector-based therapies are administered directly to patients, enabling targeted correction of genetic mutations at the site of disease.
While viral vectors excel in efficiency, stability, and targeting capabilities, they also come with higher production costs, more complex manufacturing requirements, and stricter regulatory oversight due to potential safety risks such as insertional mutagenesis or immune responses.
Non-viral methods, on the other hand, remain attractive for research and certain therapeutic applications where rapid prototyping, lower biosafety levels, or transient expression are preferred.
Ultimately, the choice between viral and non-viral delivery will depend on the specific goals, duration, and safety requirements of the project.
Major classifications of viral vectors
Adenovirus – Adenoviruses (Ad) are nonenveloped viruses comprised of a 36-kb double-stranded DNA genome.
Ad vectors derived from species C (Ad5) and species D (Ad26) are commonly used for therapeutic applications.
This vector forms a non-integrative episome in host cells and is ideal for efficiently producing high levels of transient expression in a broad range of cell types.
First-generation Ad vectors can accommodate 8 kb of transgene DNA, while later generations of Ad vectors allow a larger packaging size due to genomic deletions.
Ad vectors elicit strong immune responses, which can serve as both a limitation and an advantage.
The tissue specificity of adenoviral transgene expression can be guided by capsid tropism and promoter selection, and further enhanced through local delivery, capsid engineering, and serotype choice.
Adeno-associated virus – Adeno-associated virus (AAV) is a non-pathogenic, non-enveloped ssDNA virus with a genome size of 4.8 kilobases.
Recombinant AAV vectors lack the ability to integrate into the host genome, instead maintaining a stable episome that supports long-term expression of transgenes in non-dividing cells.
They exhibit low immunogenicity, making them less likely to provoke strong immune responses.
Several AAV serotypes have been determined to be effective in targeting specific tissues or disease sites for gene therapy.
AAV2 is widely-used serotype, while other serotypes are increasingly gaining clinical significance.
Retrovirus – Gamma retroviruses are single-stranded RNA viruses (~8 kb) that integrate a reverse-transcribed cDNA copy of their genome into the host chromosome.
This integration enables stable, long-term transgene expression and was foundational in early gene therapy.
Retroviral vectors are especially useful in ex vivo applications, where integration events can be more tightly controlled to minimize the risks of insertional mutagenesis.
Lentivirus – Lentiviruses (LV) are a subclass of enveloped retroviruses with a genome size of 8-9 kb.
LV vectors derived from HIV-1 can transduce both dividing and non-dividing mammalian cells, and produce stable, long-term expression of transgenes.
They integrate semi-randmoly into the host genome with a lower chance of insertional mutagenesis compared to other retroviral vectors.
They also have the ability to express multiple genes from a single construct, which can be valuable in complex gene therapy strategies.
Second- and third-generation LV vectors offer a much lower risk of generating replication-competent viruses and include self-inactivating elements.
LV vectors can transfect a broad range of mammalian cell types with low cytotoxicity and inflammatory potential.
Herpes simplex virus – Herpes simplex virus (HSV) contains a linear double-stranded DNA genome that persists in the host nucleus as a non-integrating episome.
HSV vectors, particularly those based on HSV-1, have a natural neuronal tropism, making it useful in targeted, non-replicative gene delivery to neurons.
Despite their relatively low immunogenicity, HSV vectors have seen limited use due to the widespread adoption of AAV-based systems.
Clinical applications of viral vectors
Viral vectors play a central role in clinical gene therapy and vaccine development, offering a proven platform for delivering therapeutic genes and antigens with high efficiency.
They have been successfully employed in the treatment of numerous cancers and chronic genetic disorders, including severe combined immunodeficiency, muscular dystrophy, hemophilia, ß-thalassemia, and sickle cell disease.
In addition to gene therapy, viral vectors have been widely used in both preclinical and clinical vaccine trials targeting infectious diseases such as HIV, malaria, Ebola, and SARS-CoV-2.
As the pipeline of vector-based therapies continues to expand, viral vectors remain indispensable to the advancement of gene delivery technologies.
For researchers developing gene therapies or working with viral vectors, a strong foundation begins with a deep understanding of the target gene’s biology, including its expression, regulation, and tissue-specific dynamics.
This knowledge informs rational vector design and ensures functional performance in the intended cellular contexts.
Evaluating different viral vector types and selecting optimal serotypes or pseudotypes is critical for maximizing safety, delivery efficiency, and tissue tropism.
Navigating the evolving regulatory landscape is also essential, as clinical development requires adherence to rigorous guidelines.
These include vector characterization, safety and biodistribution studies, and preclinical validation, all of which may differ depending on the chosen vector platform.
References
Estipona D. A Review of Viral Vectors in Gene Therapy. Biocompare. Published June 5, 2025. Accessed August 2025. https://www.biocompare.com/Editorial-Articles/619487-A-Review-of-Viral-Vectors-in-Gene-Therapy/
DePalma A. Viral vs. Non-viral Gene Delivery. Biocompare. Published November 4, 2024. Accessed August 2025. https://www.biocompare.com/Editorial-Articles/615521-Viral-vs-Non-viral-Gene-Delivery/
Travieso, T., Li, J., Mahesh, S. et al. The use of viral vectors in vaccine development. npj Vaccines 7, 75 (2022). https://doi.org/10.1038/s41541-022-00503-y