mRNA vaccine development for COVID-19

How can this possibly happen non-pathologically in humans? What really confuses me is... getting DNA outside the nucleus in the first place as a non-freak occurrence requires at least one of:

• Generation of loose DNA partial-transcripts while trying not to generate extra confusion during cell replication
  • If it's handled as ssDNA, that actually would leave it in a form pretty distinct from the usual dsDNA storage. And even as dsDNA, there are epigentic tags you could use to see that it's handled correctly.
  • This does have the benefit of being an "out" channel from the nucleus, not a security-vulnerability "in" channel.
  • I think I've reasoned myself to thinking this is the most likely explanation? I'll have to do more research to confirm/negate this guess.
• DNA cut-outs that permanently leave the nucleus, never to return (resulting in its absence in the next replication cycle)
  • You could come up with a clever working version made from frequently-duplicated genes with extra copies (ex: transposons), but... I've never heard of this. And it's a little evolutionarily-fragile.
  • This does have the benefit of being an "out" channel from the nucleus, not a security-vulnerability "in" channel.
• Reverse-Transcriptase (the most insidious of all viral proteins, that thing that crafts DNA from RNA)
  • Humans do have a special RT for extending telomeres, but it's rarely expressed, and if a random cell is expressing it that's a cancer waiting to happen.
  • There are retrotransposons that copy-paste themselves around the genome. But like... that's basically a nasty virus that got lazy and whose deck is short a few cards.
• Cytoplasmic replication
  • I am under the impression that this is not happening

In eukaryotes, I get the impression that DNA is usually not getting replicated out there in the cytoplasm, at least? DNA viruses usually have to do at least one of two things:

• Get themselves into the nucleus somehow (via small size and/or transport proteins)
• Carry their own replication proteins around with them in the virion, to produce those initial RNA transcripts that produce enough replication proteins for them to get by.

If the treatment involves entering the nucleus of fully-intact undamaged cells, or replicating itself (so really, either of these methods), the alarm bells in my head would be blaring.

But if it's just circles of extracellular DNA... I'm now kind of conflicted and confused? How virus-like do you have to be to make that a viable thing to do?

Some other lingering points of confusion/research:

Plants use ambient restriction-enzymes to make being a random cytoplasmic DNA a hazardous game (on the assumption that ambient cytoplasmic DNA is usually viruses, and the non-viruses will have co-evolved with the particular restriction enzymes to make this system work). I don't actually know that animals do, though. And you could always species-tailor it...

I also have no idea how efficient or inefficient extracellular transcriptase would be for producing mRNA transcripts. If it's inefficient, you might have to use a pretty strong primer, and I find myself a tiny bit concerned about that in a long-lived human therapeutic.

There are many application where targeting the delievery seems important. With vaccines the immune system should pick up the proteins regardles of which cell produces them. There still seems to be some complexity to it as far as the CureVac press briefing indicates. They argued that this is a time where they need to have the founder of the company lead it instead of a rather newish CEO because of the complexity.

mRNA vaccines are new technology. CEPI (Coalition for Epidemic Preparedness Innovations) was in the past investing money in the technology because of a hope of it being faster. Existing mRNA vaccines did pass phase 1 trials but there's no mRNA vaccine yet that's passed a phase 3 trial.

It's not clear how it affects that actual testing time. Actual testing time will depend a lot on how the actual effects in vivo will be.