Bottom Line: It’s allowed only if it’s a compelling safety measure, such as in certain healthcare settings, there are no less intrusive alternatives and it’s carried out as narrowly as possible.
Here’s a detailed analysis: https://ohsinsider.com/can-you-require-workers-to-get-the-covid-19-vaccination/ There’s also a Model Mandatory Vaccination Policy on the OHSI site if you do decide to go in that direction.