Messenger RNA vaccines – such as the Pfizer or Moderna COVID-19 vaccines – use lab-created mRNA to teach cells how to make a protein that mimics the protein coronavirus causes cells to make when infected.
This triggers an immune response in the body, which produces antibodies designed to protect against the coronavirus the vaccine mimics.
That means, if a vaccinated person is infected with the coronavirus, it already has the antibodies designed specifically to fight it.
However, emerging variants such as the Omicron variant have some mutations in the spike protein, which it uses to latch on to cells.
source: express.co.uk