mRNA can be used as indirect vaccine, by manipulating cellular factories to produce a foreign object that draws the attention of the immune system.

There is no known pathway of persistence in this mechanism, and there is good reason to believe such a mechanism cannot exist as it would be a highly versatile attack vector and viruses would have evolved to be much more virulent by using it. Retroviruses do complex work to infiltrate the DNA repository of a cell, if all it took is to drop some mRNA it would have been exploited and 'patched' a billion years ago.