Religion is a protected class, so employers can't discriminate against someone because of their religious background. Employers have to make accomodations. I posted earlier that accomodation could be unpaid leave, and an employee doesn't have to like the accomodations, but it's a protected class, and any employer that tries to challenge here opens themselves up to a lawsuit.
Those people obviously haven't tried to get a shot when they've had a previous allergic reaction to a vaccine. I have, and it's no picnic. The only good thing about this shot is that there's three different ones available. I can't medically get J&J or Pfizer; I had to get Moderna.
Maybe not for this shot, but there are definitely medical reasons for other vaccines why you shouldn't get it. For instance, there are vaccines I will never ever be able to get because there's too much of a risk of an allergic reaction (I actually had a medical exemption for a couple of shots in college, and if I do get vaccines, I absolutely cannot get more than one at a time). It comes across as very presumptuous to say "Oh there's absolutely no medical reasons not to get it". And I'm very, very pro vaccine.