The updated bivalent vaccines work in the same way as the original vaccines, by preparing the body to defend itself against COVID-19. Each vaccine contains molecules called mRNA, which have instructions for making the spike proteins of the original SARS-CoV-2 and the Omicron subvariant. The spike protein is a protein on the surface of the virus, which the virus needs to enter the body’s cells, and can differ between variants of the virus. By adapting vaccines, the aim is to broaden protection against different COVID-19 virus variants.

When a person is given one of these vaccines, some of their cells will read the mRNA instructions and temporarily produce the spike proteins. The person’s immune system will then recognize those proteins as foreign and activate natural defences — antibodies and T-cells — against them.

If, later on, the vaccinated person comes into contact with the virus, the immune system will recognize the spike protein on its surface and be prepared to attack it. The antibodies and immune cells can protect against COVID-19 by working together to kill the virus, preventing its entry into the body’s cells and destroying infected cells.

The mRNA molecules from the vaccines do not stay in the body but are broken down shortly after vaccination.