In that once mRNA is read it is destroyed by the cell's machinery. This takes time, but should be a exponentially decaying curve. Thus you'd need to keep inject

In that once mRNA is read it is destroyed by the cell's machinery. This takes time, but should be a exponentially decaying curve. Thus you'd need to keep injecting mRNA to maintain the effects. With the ModeRNA vaccine, due to the nuances of the immune system, antibodies will stick around. But with other therapies it may be very limited in time. Again, super big caveats to most of what I've been saying, as it's a large overview and there are exceptions to everything here.