Nature reports on the first regulatory approval in Japan of a self-amplifying RNA (saRNA) vaccine. The ARCT-154 vaccine includes mRNA coding for the familiar SARS-CoV-2 spike protein, but also codes for a replicase enzyme which allows further replication and synthesis of the SARS-CoV-2 RNA. This means a much lower dose can initially be given to recipients to elicit the desired immune response - potentially resulting in fewer or less severe side effects. The saRNA acts in some ways like a virus, although of course it is not infectious.
Many other saRNA vaccines are in development for various conditions, including HIV, influenza, rabies, and some cancers. There is even the possibility of modifying the technology to produce therapeutic proteins inside the body, as a substitute for enzyme replacement therapies.
In view of the huge boost given to RNA vaccine technology stimulated by the SARS-CoV-2 pandemic, it will be interesting to see how the saRNA field develops in future.