It wouldn't work in quite the same way. The advantage with mRNA vaccines is basically as soon as you have the sequence of the virus you can quickly introduce an

It wouldn't work in quite the same way. The advantage with mRNA vaccines is basically as soon as you have the sequence of the virus you can quickly introduce any part of it to someone who hasn't been infected yet with no risk of infecting them. This allows their immune system to get a practice run in, offering a substantial immunity to the virus (the mRNA vaccines do not code for antibodies, your body figures out the antibodies from the parts of the virus coded for in the injected mRNA). It doesn't work at all if they are already infected because their immune system is already fighting the real thing. If someone is already suffering from cancer, their immune system is already fighting against it and just showing more bits of the cancer to the immune system isn't likely to be super useful (usually cancer which is causing symptoms has already managed to evade the immune system somehow). For mRNA to be a useful therapy you'll need to do something cleverer to induce a useful immune response (or to affect the cancer cells).