mRNA is several degrees removed from the antigen your body immunizes against. Your body expresses the mRNA with less than 100% accuracy, and a vaccine manufacturing plant can't purify that difference. Your body also uptakes mRNA with less than 100% efficiency. The additional degrees of separation between biotech QA/QC, and your immune reaction forces a tradeoff: underdosing the vaccine somewhat, or risking more adverse reactions (higher dose to compensate for body's non-zero rate of mRNA mis-expression and other sources of variability).
The advantage of mRNA vaccine apparently is a few weeks faster development time. And it's flashy new tech.
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