What are viral vector vaccines and how do they work?

How do viral vector vaccines work?

Scientists began creating viral vectors more than 40 years ago.1 Viral vector vaccines use a modified version of a virus (a vector) to deliver genetic instructions to the body’s cells. The cells then produce harmless pieces of the virus called antigens which trigger an immune response in the body.

If you are exposed to the real virus later, your immune system will recognise it and know how to fight it. Vaccines which use a viral vector have been approved for the prevention of Ebola and COVID-19 and are in development for the prevention of other diseases, including malaria, influenza and HIV.1,2,3,4

How does the immune system respond to viral vector vaccines?

The immune response includes the production of specialised cells called B-cells and T-cells. B-cells produce antibodies which either attach to a virus and prevent it from infecting cells (neutralisation) or tag a virus for destruction. T-cells recognise and destroy the body’s cells that have been infected by a virus to stop them producing more viruses. Some B-cells and T-cells also work as ‘memory cells’ which enables the immune system to respond quickly and effectively if the body ever encounters the virus.

How do antibodies protect the body?

Antibodies are a key component of the body's immune response to a virus. Antibody tests can be used to detect whether the immune system has produced antibodies against a virus after natural infection or vaccination. Everyone’s immune system is different and responds differently to viruses – some people who have been infected by a virus may not have antibodies.5

Monitoring anti-vector antibodies

Viral vector vaccines do not contain any genetic instructions that allow them to replicate using the body’s cells. Even so, the body may have antibodies or produce antibodies that target the viral vector.

It’s important the body doesn’t recognise the vector as an intruder before the vaccine has had the chance to work. Measuring anti-vector antibodies over time can provide information about how well the viral vector is able to deliver instructions to the body’s cells. A low anti-vector immune response is important to help ensure that the instructions to produce the antigen are delivered and the body is able to generate a strong immune response against the virus.


1. CDC. Understanding Viral Vector COVID-19 Vaccines. Available at https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/viralvector.html. Last accessed June 2021.

2. CDC Ebola Vaccine: Information about Ervebo. Available at https://www.cdc.gov/vhf/ebola/clinicians/vaccine/index.html. Last accessed June 2021.

3. WHO Ebola virus disease: Vaccines. Available at https://www.who.int/news-room/q-a-detail/ebola-vaccines. Last accessed June 2021.

4. Vrba S et al. Development and Applications of Viral Vectored Vaccines to Combat Zoonotic and Emerging Public Health Threats. Vaccines. 2020; 8,680. doi:10.3390/vaccines8040680

5. NHS. Antibody testing to check if you've had coronavirus (COVID-19). Available at https://www.nhs.uk/conditions/coronavirus-covid-19/testing/antibody-testing-to-check-if-youve-had-coronavirus/. Last accessed June 2021.