An employer can be liable if an employee comes into work sick and exposes people to the illness. Likewise, an employer can be liable if one of its employees is an asymptomatic spreader of a communicable disease like hepatitis, and exposes someone to it.

An employer can be liable if one of its employees is an alcoholic and causes damage while drunk on the job.

OBVIOUSLY, THESE SITUATIONS ARE NOT ANALOGOUS, because the risks of coming into work sick/drunk are far greater than the risks of coming into work without a vaccine. Pro-vaxers will argue otherwise, and I hope courts swiftly shoot that logic down. Just throwing out some examples of how personal health choices can affect people performing their duties at work, since that's what you mentioned.