Yes, employment contracts like teachers, health and many professional can require vaccines, testing, etc. Unfortunately a low wage call center does not provide an employee contract. Yes, the call center could have fired them, but they were desperate for employees. One CC had a death, CDC and VOSHA were involved.
Under Virginia law, they would be required to pay unemployment, for lack of "just cause", if fired. Virginia is also, employment "at will".
I found several articles, one attached that the State of CA is an employement "at will" state.
Employment at Will: What It Really Means in California
The following is excerpted from Chapter 2 of
California Employment Law: An Employer's Guide, Revised and Updated for 2017 (SHRM, 2017), written by James J. McDonald, Jr., a managing partner at the labor and employment law firm Fisher Phillips.
California's Labor Code contains a presumption that employees are employed at will. This means that either the employer or the employee may terminate employment at any time, with or without cause or prior notice. This is important for employers because "cause" is defined under California law as "a fair and honest cause or reason, regulated by good faith on the part of the employer." Employers would be significantly burdened if they had to prove to a court or jury that they acted "fairly" and "in good faith" in every employee termination.
California Code...
Codes Display Text.
2922. An employment, having no specified term, may be
terminated at the will of either party on notice to the other. Employment for a specified term means an employment for a period greater than one month.