mRNA is only one of the various classes of vaccines that are approved for Covid. It's also been something that has been under development since SARS, and has been proposed as a drug-delivery mechanism for other diseases (think it's being tried for AIDS).

If one didn't want mRNA vaccines, there are others that older mechanisms for delivering inactivated viruses, protein fragments, etc.

EDIT: it's sorta a dream to have the patient manufacture the drug matter, particularly if it's manufactured in a particular site in the body. This could have applications in cancer treatments or for localizing side-effects. Something that only appears in the screwed-up environment of cancer cells beats the snot out of dosing something that happens to kill the cancer cells faster than the patient.