mRNA does not last in body more than a few weeks. I am more worried about my childhood chicken pox virus making a outbreak in form of shingles. That virus is known to hide in neck bone marrow for as long as 6-7 decades.
Even more scary was the smallpox I had at age 8 growing up in India. Smallpox has a kill ratio 30 times higher than covid-19.
mRNA half-life is about 10 hours (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC403777/) so even if the vaccine contained 100 million mRNA strands, they’ll all be gone in a week.