It is absorbed by lymphocytes (b-cells specifically) and is broken down and destroyed
The B cells are too buy reacting to the spike protein.
The mRNA is either degraded within the cell - with a Pfizer/Moderna trying to keep it stabilized longer than it normally would be, for a few more rounds of transcription by the ribosomes.
or
Any mRNA that escapes from a ruptured cell gets degraded rapidly by RNases in the blood stream.
Any mRNA from the vaccine, and the spike protein fragments are gone in approx 10 days. But one of the anti-VAX conspiracies says they stay around forever. Oh my.
I'm been waiting, waiting for the anti-VAX conspiracy that mRNA changes the human DNA. That's always a fun conspiracy.
The Jewel of the Mind is Colored with the Hue of what it Imagines