Even if they don’t circulate this will still supposedly destroy the ones attached to cells, arteries and capillaries. And keep everything from attacking itself.
When a cell produces alien stuff (convinced by a virus or mRNA vaccine to do this), it detects this stuff - part of our self defence. The cell then emits stuff that alerts immune system cells. Killer cells move in, destroy the cell, and eat up the mess. This suicide-by-cop mechanism makes sure that there are not many spike proteins that escape. A virus can suppress this mechanism to replicate undetected for a long time, a vaccine can't.
Escaping mRNA cannot live long, maybe it destroys a cell before it disintegrates, but destroyed cells happen all day long and the body knows how to deal with it.
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