“Vector-based” means using a harmless virus as a delivery vehicle to carry genetic instructions into your cells. The term comes up most often with vaccines and gene therapies, where scientists need a way to get a specific piece of genetic code inside the body so cells can read it and produce a desired protein. The “vector” is the modified virus doing the delivering, not the treatment itself.
How a Viral Vector Works
A viral vector starts as a real virus, but scientists strip out the parts that make it dangerous. In many cases, they delete the genes responsible for replication entirely, so the virus can enter your cells but can’t copy itself or cause infection. What’s left is essentially an empty shell with a talent for getting inside cells, which is what viruses naturally do well.
Scientists then load new genetic instructions into that shell. For a vaccine, those instructions typically code for a single protein from the pathogen you want protection against. For a gene therapy, the instructions might replace a faulty gene with a working copy. Once injected, the vector enters your cells and deposits its genetic cargo in the nucleus, where the cell reads the instructions and starts producing the target protein. Your immune system then recognizes that protein and mounts a response, or in the case of gene therapy, the newly produced protein restores a missing function.
The Oxford-AstraZeneca COVID-19 vaccine is a good example. It used a modified chimpanzee adenovirus to carry the gene for the SARS-CoV-2 spike protein into cells. Once inside the nucleus, that gene was transcribed into messenger RNA, which traveled out to the cell’s protein-making machinery. The spike proteins produced were then displayed on the cell surface, training the immune system to recognize and fight the real virus.
How Vectors Differ From mRNA Vaccines
Both vector-based and mRNA vaccines give your cells instructions to make a specific protein. The difference is in the delivery method. An mRNA vaccine wraps its instructions in a tiny fat bubble (a lipid nanoparticle) and sends them directly into the cell’s protein-making machinery, bypassing the nucleus entirely. A vector-based vaccine uses a modified virus to carry DNA instructions into the nucleus first, where they’re converted to mRNA before protein production begins.
This distinction matters practically. Vector-based vaccines have historically been easier to store, often remaining stable at standard refrigerator temperatures (2 to 8°C) for months or even years when freeze-dried. Early mRNA vaccines required ultra-cold storage, though that gap has narrowed with newer formulations.
What Makes Vectors Safe
The key safety feature is that viral vectors are replication-deficient. Scientists achieve this by deleting essential genes the virus would need to copy itself. In adenovirus vectors, for instance, critical early gene sequences are removed so the virus can still enter cells and deliver its payload but cannot reproduce. In vectors built from adeno-associated virus (AAV), essentially the entire original viral genome is deleted except for two small noncoding regions needed for packaging. What remains is a delivery system, not an infection.
Additional safety modifications include removing genes that help viruses evade the immune system and deleting genes that prevent infected cells from self-destructing. These changes mean the body can clear the vector quickly after it has done its job.
Types of Viral Vectors
Different viruses serve as vectors depending on the goal. Each has trade-offs in how much genetic material it can carry, how long it stays active, and which cell types it reaches best.
- Adenoviruses are among the most widely used vectors for vaccines. They efficiently enter many cell types and trigger strong immune responses. The Johnson & Johnson COVID-19 vaccine and the Ebola vaccine Ervebo both use adenovirus-based platforms.
- Adeno-associated viruses (AAV) dominate gene therapy. They’re small, easy to produce, rarely integrate into your DNA (reducing the risk of unintended genetic changes), and have never been linked to human disease in their natural form. Since 2021, five of the seven new FDA-approved gene therapies have used AAV vectors.
- Lentiviruses can carry large genetic payloads (up to about 12 to 15 kilobases of DNA) and integrate stably into the host genome, making them useful when long-term gene expression is needed. They work in both dividing and non-dividing cells and produce the target protein faster than AAV vectors.
- Herpes simplex viruses (HSV) have been adapted for cancer treatment and gene delivery to the skin. The melanoma therapy IMLYGIC and the skin disorder treatment Vyjuvek both use modified HSV-1.
Pre-Existing Immunity Can Reduce Effectiveness
One challenge with vector-based therapies is that your immune system may already recognize the delivery virus from a past natural infection. If you’ve previously been exposed to the same type of virus used as the vector, your body may neutralize it before it can deliver its genetic cargo. Research on AAV vectors found a direct correlation between the level of pre-existing antibodies against the virus shell and the degree of reduced effectiveness. In some cases, even antibody levels too low to detect with standard lab tests still partially blocked the vaccine’s effect in animal studies.
This is one reason scientists use uncommon virus strains as vectors. The Oxford-AstraZeneca vaccine, for example, chose a chimpanzee adenovirus rather than a common human one, reducing the chance that people would already have immunity to it. Researchers have also found that newer AAV serotypes (like AAV7 and AAV8) resist neutralization by human antibodies roughly 30 times better than the more common AAV2.
Approved Therapies Using Vectors
Vector-based technology has moved well beyond COVID-19 vaccines. As of 2024, the FDA has approved vector-based treatments for conditions ranging from inherited blindness to muscular dystrophy. Luxturna, approved in 2017, uses an AAV2 vector to deliver a working copy of a gene to retinal cells in people with a rare form of inherited vision loss. Zolgensma, approved in 2019, delivers the missing SMN1 gene to treat spinal muscular atrophy in young children using an AAV9 vector given as a single intravenous infusion.
More recent approvals include treatments for hemophilia A and B, where a single infusion delivers the gene for the missing clotting factor, potentially replacing a lifetime of regular injections. Elevidys, approved in 2023, delivers a shortened version of the dystrophin gene to treat Duchenne muscular dystrophy. And in 2024, a treatment for a rare enzyme deficiency affecting the brain became the first vector-based therapy delivered directly into the brain tissue.
These approvals reflect a broader shift: “vector-based” increasingly refers not just to vaccines but to a growing category of one-time gene therapies designed to correct genetic diseases at their source.