"The technology behind the new COVID-19 mRNA vaccines" is the title of a video that is 1 hour 25 mins.

22:56 minute mark the expert on lipid delivery of mRNA begins to describe how the entry in order to vaccinate a cell will cause it to generate spike proteins which the cell will automatically "hold out" for other body immune system cells to identify.

One of these is a cyto-toxic T-cell which will carries a piece of (spike protein) antigen to match to other cell and kills them, such as a Covid infected cell.

52;36 mark someone asks "hey well if the vaccinated cell in holding out the (spike protein) antigen also will the cyto-toxic T-cell attack and kill that also" PAUSE - "Hmm (name) do you have an answer for that?" "We need an immunologist" "I don't THINK so because different cells hold out the antigen (spike protein/foreign matter) differently.

https://www.youtube.com/watch?v=lJO51Sguyaw