I think it's actually the mutagen Molnupiravir from Merck that substitutes for some nucleotides during RNA replication and then causes subsequent replication failure. The RNA protease inhibitor Paxlovid from Pfizer does not seem to interact with any human biological process. It just inhibits the SARS-COV2 protease that cuts the big super protein into the smaller functional proteins the virus needs to replicate. Humans don't make a protease that Paxlovid could interact with, and indeed we don't even have the DNA or RNA sequence the SARS-COV2 protease cleaves. Which is the reason Paxlovid is so benign for humans that Pfizer's toxicity test could find no upper limit.

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