I had been wondering about this, and a fellow coworker who is on leave (I can't confirm what kind of leave my coworker is taking) because of a surgery. They received Karen's email in the actual mail (more or less) telling them, even though they are on leave, vax proof is still required.
I am only posting this to let people know. I’m sure those letters arrived to a lot of other HW employees on leave right now too.