The general principle is the same as for most vaccines: make the body generate its own antibodies (and/or T cells) for the virus in question. So in that sense of course its effect is long term, just like when you get sick with some virus and your body develops antibodies for it - the virus itself is gone, but the effect on the immune system is long-lasting.
From an FT article:
> Conventional vaccines administer an inactivated virus, or proteins from that virus, into the body to trigger an immune response, which can prevent subsequent infection.
> In contrast, mRNA technology — originally developed as a cancer therapy — injects genetic instructions into the body that tell cells to make viral proteins that prime the immune system. Although mRNA vaccines had been under development for several years for viruses including influenza, cytomegalovirus, HIV, rabies and Zika, the arrival of Covid-19 turbocharged the process.
- https://www.ft.com/content/74e41528-80c3-4b0f-b343-be43d90f0...
So yes, but no. And it's reassuring compared to say DNA tweaking which would permanently change the instructions in some cell lines in your body, or more extremely germ line DNA tweaking which would alter your cells and those of any subsequent offspring you have.