@Neonsignal - The payload mRNA carries works almost-exactly as does an actual virus, with one exception -- what it produces cannot replicate because the part necessary to do that is missing.

When a virus gets into the cell the RNA in the virus "programs" the cellular metabolism to produce copies of itself. This is "replication competent" and after some number of copies, usually in the range of 1,000 to 10,000 is produced, the cell is destroyed and the copies released. Each is capable of infecting a new cell, thus it is replication-competent.

mRNA works the same way that a virus does, EXCEPT that unlike a virus the mRNA itself contains the replication instruction and the instructions are intentionally defective, causing only part of the whole virus to be produced. For this reason what gets produced is NOT replication-competent. So the cell takes up the mRNA, the cellular machinery produces the copies as programmed in the mRNA, but what's produced cannot cause more copies to be made since what's made is incomplete. In this case what's programmed to be made in the mRNA is the "S" or spike protein.

Those copies, while they can AND DO bind to cells in the body exactly as does the actual virus (thus producing the pathology and also triggering the immune response) they cannot cause another replication event (and thus go exponential) because the other part of what's necessary for them to do so (the "N" or nucleopcapsid section) is intentionally missing from the mRNA instructions.

Note that in a viral infection MOST of the virons produced do NOT successfully invade a new cell. IF they did you'd die every time because viral replication is exponential and within a few days every single cell in your body would be infected and destroyed! In addition you couldn't pass the infection to anyone else because every viron would invade a new cell in your body; there would be none to expel in your breath, cough, feces, blood, semen, etc.

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