SARS-CoV-2 viruses infect people using a surface protein, called Spike, which acts as a key allowing viruses to enter cells, where they can then reproduce. All vaccines currently under study have been developed to induce a response that blocks the Spike protein and thus prevents cell infection. The two COVID-19 mRNA vaccines approved for the vaccination campaign use messenger ribonucleic acid (mRNA) molecules that contain instructions for the cells of the vaccinated person to synthesise Spike proteins.

The proteins produced stimulate the immune system to produce specific antibodies. In those who have been vaccinated and are exposed to viral infection, the antibodies thus produced block the Spike proteins and prevent their entry into the cells.

Vaccination also activates T-cells that prepare the immune system to respond to further exposure to SARS-CoV-2.

The vaccine, therefore, does not introduce the actual virus into the cells of the vaccinated person, but only the genetic information that the cell needs to build copies of the Spike protein. If, at a later date, the vaccinated person comes into contact with SARS-CoV-2 again, his or her immune system will recognise the virus and be ready to fight it.

The vaccine mRNA does not remain in the body, but degrades shortly after vaccination.