And some workers are now suffering the consequences of their defiance.
United Airlines announced Wednesday the firing of 593 employees after they required all 67,000 workers to be vaccinated by the end of September.
"All of our employees recognize this is not an easy decision. There was the potential for blowback and criticism of the company as a result, but I hope that they are just as gratified as we have been that the compliments and support for this policy far outweighed the criticism that we've received," said Josh Earnest, the COO of United Airlines, on Good Morning America.
Earlier this month, President Biden announced that his administration will require private businesses with 100 or more employees to enforce a vaccine mandate or require weekly testing.
Pilots unions for American and Southwest are asking the White House for exemptions to the vaccine mandate, warning it could impact holiday travel.
In a letter to federal officials the Allied Pilots Association, the union representing 14,000 U.S. pilots, defended member's concerns.
"Some of APA's members are unable to undergo vaccination for documented medical reasons, while others are reluctant to get vaccinated based upon concerns about the potential for career-ending side effects. All of those members are still able to perform their duties as professional pilots. To force those pilots out of their positions, rather than offering viable alternatives will have adverse consequences upon their families and the airline industry," the union said.
Seven United employees are suing the airline over the mandate.
Villanova University Law School Professor Ann Juliano, who specializes in employment law, explained to 6abc what rights employers and employees have in this scenario.
"In general, people think they have more rights than they do in the workplace. I would believe in most cases, that is a private employer and not a governmental employer, that the employer is going to be successful in these lawsuits, particularly now that the vaccines are going to be fully approved by the FDA," Juliano said.
She also said, "Employers need to know just because they can do something doesn't mean they have to."
Philadelphia International Airport officials say United Airlines does not have enough of a presence here to have an impact regarding Wednesday's announcement. However, some airports are anticipating protests as deadlines come and go.
Healthcare systems have also hit deadlines. St. Luke's University Healthcare Network is focusing on the fact that 99% of their employees are vaccinated. While they are disappointed that 155 full and part-time staff members opted to not get vaccinated, Senior Vice President of Human Resources Evan Ochs said it will not impact patient care.
"Although we didn't want anyone to leave, and we were hoping that 100% of employees would comply, obviously that wasn't the case. Fortunately, we had less than 100 full-time employees who did choose to resign. When you look at over 17,000 employees in total that was less than one percent of our workforce," said Ochs.