The immune system identifies and destroys cells containing foreign mRNA because they produce foreign proteins.

“And even if it were to hang around, the purpose of the immune system is to find these cells with this foreign mRNA in it because it makes proteins that are foreign and to destroy those cells. That's why you get an immunogenic response when you get the flu vaccine or in this case, the mRNA vaccine, you get pain at the site, you get a little bit of a fever, perhaps, you might have some muscle aches, body aches, it'll feel like you actually have a viral infection because that's your interferon that's responding to it.”