How does the Pfizer vaccine work?

The Pfizer vaccine is a messenger RNA (mRNA) vaccine. The messenger RNA instructs cells in our body to make a harmless piece of protein called the viral spike (S) protein. This spike protein is the one found on the surface of the virus that causes COVID-19 (the SARS-CoV-2 virus). The spike protein helps the virus get into our cells.
The protein triggers the body’s immune system to produce a neutralising antibody and other immune responses to fight against it. This means our body develops a ‘memory’ of how to protect us if we do become infected with the virus.
The cell breaks down the mRNA soon after it has done its instructing job.