Virus variants can evade both vaccine immunity and immunity from prior infection.

Applies even more so to therapeutics like monoclonal antibodies and antivirals as well, which don't have nearly the diverse immune response as vaccines.

We've already seen both. New variants have high rates of reinfection, and a number of therapeutics have seen significant evasion.

Prophylaxis from vaccination is always preferred to prevent mutations to begin with, and in the case of infection to reduce total viral replication and thus mutation risk.