“A self-amplifying mRNA shot, as the name implies, contains the equipment needed to make more of itself once it enters cells. You do this by not only injecting the mRNA for the antigen of interest (such as one that encodes the coronavirus spike protein) but also mRNAs that get translated into replicase proteins that will in turn produce more of the mRNA species. Picture sending someone a sheet of paper with some important information on it, and then imagine that you’ve sent them a whole pile of copies of that sheet so they can distribute them. Now imagine sending them a bunch of sheets of material that can assemble themselves into a working photocopier and crank out more sheets when they do. That sounds weird and ridiculous, but hey, that’s biology for you. It’s very, very strange down there in the cell.
Having an mRNA that can make more copies of itself means that (first off) you probably don’t have to inject very much. John von Neumann, an early thinker in the field of self-replicating devices, would have been beside himself with excitement. It also means that even with a small initial injection that you might well expect to get a larger and more thorough exposure to your desired antigen protein than you could feasibly inject in a standard all-at-once dose. So although this idea didn’t make it into the first round of coronavirus vaccines, it has definitely not gone away, and there have been reports of progress in the field.”