N1120A
Pattern Altitude
Not much at all. Unless they rise to the level of having a disability (and that may be a hard one for flight crews to argue), they don't get any mandated accommodation. Even with ADA involved, if the employer can argue it is essential to the job, they can deny the accomodation.
Eh, no one is going to win an argument that a vaccination is so essential to rise to failing to accommodate an actual disability. Plus, they could ultimately accommodate by scheduling in a manner consistent with countries that allow for such accommodations. However, given how rare there are legitimate medical reasons to not take a vaccine, it would probably affect literally 1-2 pilots at United.