Understanding Immunogenicity

Scientists and researchers all over the world are continuing to assess and evaluate the level of protection COVID-19 vaccines will have over time.

Vaccine clinical trials are designed to build an understanding of the safety of the vaccine and to determine how well they work, their efficacy and immunogenicity, across different populations.1

What is immunogenicity?

Immunogenicity is different to efficacy in that it is a more complex measure of how well a vaccine works and measures the ability of a vaccine to produce an immune response, and guides how long that response will last over time. Efficacy is a measure of how well a vaccine works and can be measured by investigating a vaccine’s ability to prevent disease.1

Immunogenicity studies work out what type of immune response a vaccine or disease will trigger and how long it will last.2  These studies allow scientists to decide how suitable a vaccine will be, or is, against different types of viruses. They can also be used to determine the right dose of a vaccine, as well as if and when booster vaccinations may be required.1 Understanding the relationship between the immune response and protection from a disease is important to know how effective a vaccine will be.

How does the immune system work?

The immune system is the body’s biological defence to protect against infection caused by foreign invaders, such as viruses. As a virus infects the body, it will be detected by the immune system – the ‘detect and destroy’ protection force of the body.

Immune systems have evolved to be able to detect previously unencountered invaders (e.g., viruses).

What’s the role of B-cells and T-cells?

The body’s immune system (made up in part of B-cells and T-cells) is generally very good at identifying and destroying intruders (viruses).

Immunogenicity for vaccines

Vaccines work by prompting or priming the body’s own natural defence system. They teach the body to recognise intruders by introducing it to either a part of, or an inactive form of the virus. After vaccination, if the body then encounters the intruder, its immune system is ready to fight back more quickly and effectively.3  Vaccines generating both B-cell and T-cell responses give the immune system a two-pronged response to fight infection.

Vector vaccines use a modified version of a virus (a vector) to deliver important information to prime the body’s immune system. Vectors can be modified viruses (as in the case of COVID-19 Vaccine AstraZeneca) to enable the 'cargo' i.e., spike protein genetic information, to get into the body's cells.

For viral vector vaccines, it is 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 that the viral vector is still able to deliver the essential ingredients to protect against the disease and generate immunity.

How is immunogenicity measured?

Antibodies are thought to play a key role in immunity to COVID-19.4  Antibodies are produced by the immune system in response to the presence of an intruder. They act like the security for the body and are produced by the immune system when it detects an intruder (e.g., a virus). When a person comes into contact with a virus they have been vaccinated against, the body can call on the immune system to react quickly with the right response (antibodies) to fight the infection successfully before it causes significant illness.

The scientific community are continuing to build a greater understanding of the immune response to the COVID-19 virus, including antibody and T-cell responses. Immune responses to vaccines are currently measured with a wide range of different tests and studies and so comparisons between different vaccines are not yet possible.2


1. World Health Organisation, Guidelines on clinical evaluation of vaccines: regulatory expectations. 2016. Available at https://www.who.int/biologicals/BS2287_Clinical_guidelines_final_LINE_NOs_20_July_2016.pdf. Last accessed May 2021.

2. Hodgson S, Mansatta K, Mallett G, et al. What defines an efficacious COVID-19 vaccine? A review of the challenges assessing the clinical efficacy of vaccines against SARS-CoV02. 2020. Available at doi.org/10.1016/S1473-3099(20)30773-8. Last accessed May 2021.

3. British Society of Immunology. How vaccines work. Available at https://www.immunology.org/celebrate-vaccines/public engagement/guide-childhood-vaccinations/how-vaccines-work. Last accessed May 2021.

4. Pecetta S, Pizza M, Sala C et al. Antibodies, epicenter of SARS-CoV-2 immunology. Available at https://www.nature.com/articles/s41418-020-00711-w. Last accessed May 2021.