@Neonsignal -- I don't think we have a clean answer to that, nor does it really matter. Without replication-competent production it's immaterial since it's a "once through" process and then is done.

The problem is that natural infection starts in the upper respiratory tract and, if it turns into a systemic infection, becomes extremely serious. Most of the time it does not, just as with other upper respiratory infections.

But a shot into a muscle intentionally is systemic on an immediate basis. That is, the mRNA will be intentionally taken up by whatever it contacts first in all probability; for an IM injection that means the epithelium of the blood vessels throughout the body, since muscles are heavily-perfused. Within seconds to a couple of minutes of the injection you will have distributed that material with most of it "landing" on the cells lining your blood vessels and those are where the spike will be produced and the antibody response occur. That's insanely bad if the spike ITSELF is pathogenic because the VERY LAST PLACE you want any sort of pathology is in the lining of your blood vessels, and we now know that indeed the spike alone is pathogenic.

Incidentally this also may mean that there is NO SUCH THING as a safe vaccine delivered as a shot for a coronavirus, since ANY introduction of the spike protein into the circulation is dangerous and likely to produce severe damage including but not limited to strokes, heart attacks and thrombus of the organs damaging or destroying them. We know most natural infections do not wind up being systemic as is evidenced from the data on professional athletes that were infected and yet very few suffered any sort of systemic or circulatory harm.

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