What prevents an employer from mandating vaccination?

Question

IANAL: Would said requirement need to be found unconstitutional in order to prevent mandating vaccine in prisons, where prisoners are literally a captive audience? The two stakeholders comprise at least prisoners & guards. What legal hurdles would such a vaccination mandate face?

CONTEXT: The headline US prison guards refusing vaccine despite COVID-19 outbreaks indicates: Prison guards are refusing coronavirus vaccines at alarming rates..

UPDATE: September 2021: "My job as president is to protect all Americans," Mr. Biden said Thursday. "So tonight, I'm announcing that the Department of Labor is developing an emergency rule to require all employers with 100 or more employees that together employ over 80 million workers to ensure their workforces are fully vaccinated or show a negative test at least once a week." reference link / source

Answer 1

One thing that may prevent this is contract law: the employer may not have the power to impose new requirements on employees during the period of the current contract. For prisons that are run by government agencies, there may also be statutory restrictions on what the warden or Bureau of Prisons can compel employees to do without legislative approval (this is a general feature of government employment). There are additional disability and religion-based protections for employees. Apart from such legal considerations, the vaccine is not universally available, which explains why not all employers mandate that employees get vaccinated. It's not clear how prisons, specifically, are relevant: there's no general rule that "because it's a prison, normal law is suspended".

Answer 2

Reasonableness

An employer is entitled to issue lawful and reasonable directions to its employees and failure by an employee to comply is grounds for disciplinary action. Conversely, an employee is not required to follow unlawful or unreasonable instructions.

Mandating vaccination is, AFAIK, lawful.

So the question is: is it reasonable?

Reasonableness requires consideration of all the circumstances. On the one hand, there is a requirement for invasive medical intervention, albeit, a relatively minor one. On the other are the particular risk to the employee, other employees and the employer's operations if the employee contracts Covid-19 and other methods that could be used to manage that risk.

This may mean that the instruction might be reasonable for some employees (e.g. those working in the prison hospital) but not for others (e.g. administrative staff with no prisoner contact).