Requisition (symptoatic or asymptomatic), or simply acquiring viral particals and re-distributing them, are both possible.

It's also far easier to establish (and verify) a "masks requried" policy (visual inspecition is sufficient) than a "masks or fully-completed vaccination schedule required" one.

The ultimate goal isn't protecting individual health but public health, and here, social measures including distancing, quarantine, masks, sanitation (though perhaps less so than first thought), etc., all reduce the probability of transmission through the population at large.

We're used to thinking of individual health. Public health is related, but is not the same thing. Logics and measures that are appropriate or sensible in the case of individual health may not be for public health. (And vice versa.)

Public health is an emergent phenomenon of individual health.