I don’t think this is nearly as easy a question to answer as the other replies suggest. Pfizer will be required to make the case for safety whenever they apply for approval. It is premature to discuss this without reviewing the actual evidence they put forth.

That being said, one of the reasons that protease inhibitors are considered the way to go is that “no human proteases with a similar cleavage specificity are known, such inhibitors are unlikely to be toxic.” [1]

[1] https://www.science.org/doi/10.1126/science.abb3405