If I understand mRNA COVID vaccines correctly, they contain some RNA that is inserted into a cell, which then is instructed to produce the spike protein, which is typical for COVID-19 viruses. Once the body has produced an immune response to the spike protein, it is better equipped to fight the actual COVID virus. So far, so good.
How do we know that the cells that have spike proteins on them do not pose a threat to the body? It seems like such cells have some sort of protuberance that might interfere with all sorts of cell activity that require close cell interaction.
Moreover, how does the cell (or the body) know when to stop producing spike proteins? Does the mRNA just run out at some point and there is none of it left to enter new cells?