system
Given the following post, respond as if you're commenting on Reddit:
human
How long does the DNA of viral vector vaccines remain in the host cell and for how long would the cell execute its instructions?

I apologize if this question has already been asked, i could not find any threads or concrete answers about this (or maybe i just didn't understand them :).

So, i've been reading about the mechanism of the new Adenoviral vector vaccines (AZ, J&J, etc). From all explanations i've read, this is my (very simplistic) understanding:

1. Modified adenovirus enters the cell
2. DNA is injected in the nucleus
3. mRNA is "generated" and exits the nucleus
4. Cell starts producing and expressing spike protein
5. Immune response is provoked

What i'm wondering is, basically, what exactly happens with the cell after that:

Does the viral DNA degrade at some point?

Once this DNA is in the cell's nucleus, for how long is step 3 executed?

Since those vaccines are administered in the muscle, i'm guessing it's exactly the muscle cells that get 'infected' and start expressing the spike protein.  And from a very quick search, it seems that the lifetime of a muscle cell is 10-15 years. 

I guess a bonus question would be, once the cell expresses the spike protein, does the immune system kill it or persistently continues trying to? In both cases, it doesn't seem like a great deal for a cell that's supposed to live that long :)
gpt
I don't believe (and may well be entirely wrong) that the length of time that the viral DNA persists in infected cells is well characterized. Infected cells are likely killed by the immune system before the viral DNA is degraded. In an adenovirus vaccine platform the replication of the virus has been attenuated so that it does not produce new virus. One problem with many DNA viruses is that the viral DNA can be incorporated into the host DNA. This is a low frequency event and the relatively small amount of virus (compared to an active natural virus infection) may make it a very unlikely event. If this does occur, though, a properly functioning immune system would destroy the cell if it caused transformation or if it incorporated and caused the cell to produce viral proteins. As for the loss of  cells due to a functioning immune system - this is trivial. There just aren't enough infected cells to make a significant difference in the tissue.