With the highly contagious COVID-19 Delta variant on the rise, mandatory vaccination requirements are becoming more prevalent in the workplace. Unless prohibited by law or a collective bargaining agreement, employers are free to determine the terms and conditions of work, including that vaccination against COVID-19 is an essential safety rule and qualification for employment. Those who object to getting the vaccine are responding by increasingly seeking exemptions from mandatory vaccination based on their religious beliefs. Of the three possible responses by an employer—approval, rejection on the ground the employee's religious belief is not credible, or that honoring the request would cause it undue hardship—this article focuses on the undue hardship defense. Unlike the heavy burden on employers to accommodate a physical or mental disability, in the religious accommodation context an employer need only show that the accommodation would require it to incur more than a "de minimis cost." This lower standard may allow many employers, not subject to more protective state or city laws, to holistically challenge requests for religious accommodation to sustain the integrity of their mandatory vaccination rules and the safety of its workforces.