Sorry, but you simply are wrong. The lipid encased mRNA molecules have a half-life of around eight hours in the body and are gone completely in 10-14 days.
Fraid not. The mRNA may have a short half life ... but the polyethylene glycol coated nanoparticles can’t be metabolized. That’s what the polyethylene coating is FOR ... to prevent rapid breakdown. The particles themselves, with the coating, are too big to make it through the glomerulus, i.e. to be urinated out.
Here:
https://pubmed.ncbi.nlm.nih.gov/24929223/
is an article which states that high molecular weight polyethylene glycol generally just stays in the body.
Key quote: “High molecular weight PEGs show slow renal clearance, and consequently have a greater potential to accumulate within cells. The intracellular nonbiodegradable PEG can accumulate within the lysosome ultimately causing distension and vacuolation observed by standard histological examinations.”
Attaching the PEG to an actual particle would, of course, slow renal clearance even more, leading the nonbiodegradeable PEG, and presumably whatever that non biodegradable PEG is coating, to accumulate even more in the body.
In other words: a PEG covered nanoparticle may stay intact, slowly releasing its contents until empty.