Is the code from the mRNA vaccine executed by the cells as a one-off? [closed]

Question

Is the code from the mRNA vaccine executed by the cell that has taken it up as a one-off or does the cell produce the spike proteins endlessly (like a programming loop) until the cell is killed by the immune system?

In other words, are the mRNA instructions executed once and then discarded, the result is a single batch of whatever the mRNA is telling the cell to produce and then the cell goes back to doing its usual stuff? Or, is it the case that this cell, once it absorbs the mRNA, it then executes only this instruction forever - until recognised by the immune system and killed? If that is the case then can, hypothetically, the same cell absorb a different mRNA later on (before it is killed) and start producing something new based on that new mRNA or is it stuck with that previous mRNA until killed?

Answer 1

Basically neither of your hypotheses is correct. A piece of mRNA isn't usually destroyed after one transcription. Nor does it create an "infinite loop" in the cell. Instead there's a balance between transcription and decay. The mechanisms for mRNA decay are fairly complex in themselves. (see link for details)

For vaccines, it's obviously beneficial to delay the decay, so various "counters" to a (more) rapid decay are introduced, which increase the half-life of mRNA. For example, the Pfizer/BioNTech vaccine has a "double UTR", comprised of two sequences that were experimentally observed (in combination) to give the longest half-life from all pair combinations in a library.

Furthermore, more than one mRNA (type) can compete for translation inside the same cell.

Also, cells will usually try to recognize and rapidly decay/degrade foreign mRNA as well; again countering this was major problem for mRNA vaccine development.