How vaccines work

Vaccines work by stimulating a response from the immune system to a virus or bacterium. This creates a ‘memory’ in the immune system. This immune memory allows the body to ‘remember’ a specific virus or bacterium, so that it can protect itself against this virus or bacterium and prevent disease that it causes.

Most vaccines contain a weakened or an inactivated (killed) form of a virus or bacterium, or a small part of the virus or bacterium that cannot cause disease. This is called an antigen.

When a person gets a vaccine, their immune system recognises the antigen as foreign. This activates the immune cells so that they kill the disease-causing virus or bacterium and make antibodies against it.

It also activates immune cells - called T-cells and B-cells - in the blood, in the bone marrow and throughout the body.

Later, if the person comes into contact with the actual virus or bacterium, their immune system will remember it.

It can then produce the right antibodies and activate the right immune cells quickly, to kill the virus or bacterium. This protects the person from the disease.

Different vaccines bring about different levels of protection. How long protection lasts also depends on the disease it protects against. Some vaccines can only protect against a disease for a short period and may need booster doses; for others, immunity can last a lifetime.

Vaccination does not only protect people who have received a vaccine. By decreasing the risk of being exposed to infection, it also indirectly protects unvaccinated people in the community, such as children who are too young to be vaccinated or people with weakened immune systems.

This community immunity (also called herd immunity) needs enough people in an area to be vaccinated.

In contrast, people who become immune by getting the actual disease can:

expose other people to the disease
put themselves at risk of serious complications from the disease.

How vaccines work — 1. Antigen 2. Antibodies 3. Immune-response

Protein-based vaccines

Vaccines based on proteins contain small protein fragments of a virus or bacterium for the immune system to recognise as foreign.

They include well-established vaccines, like flu, tetanus and whooping cough vaccines.

These vaccines often contain proteins from a virus’s surface. In the virus, these proteins allow it to attach itself to a human cell and infect it. But in a vaccine, the proteins, made in a laboratory, only stimulate the immune system and do not cause an infection or disease.

Protein-based vaccines often contain substances called adjuvants. These strengthen the immune system’s response to the vaccine and increase protection.

Protein-based vaccines have been in use for many years.

More recently, the European Union (EU) has authorised new vaccines based on proteins, including a vaccine to protect against COVID-19.

mRNA and viral-vector vaccines

In place of a protein, mRNA and viral-vector vaccines contain instructions for the human cells that tell them how to make an antigen protein. These instructions come in one of two forms:

a molecule called messenger ribonucleic acid or mRNA;
genetic information inside a harmless ‘vector’ or carrier virus, modified so that it cannot cause a disease.

When a person receives an mRNA or a viral-vector vaccine, some of their cells read these instructions. These cells then produce the antigen protein for a short period of time before breaking the mRNA or the harmless virus down.

The immune system recognises the antigen protein produced by the body’s own cells as foreign, activating immune cells and creating antibodies.

Researchers have been working on developing mRNA and viral-vector vaccines for decades, making significant advances in the 2010s.

Following additional investment at the start of the COVID-19 pandemic in 2020, the first four vaccines against COVID-19 authorised in the EU were mRNA or viral-vector vaccines.

More information on these vaccines and how the EU approved them is available: COVID-19 vaccines.