It is the first major U.S. carrier to mandate vaccines for all employees. Delta Air Lines announced earlier this year that it is requiring all new employees to be inoculated.
United joins a growing list of companies that have mandated vaccines for employees in some capacity: Uber, DoorDash, Facebook, Google, Twitter, Walmart and many more.
The airline's deadline for employees to upload their vaccine card is five weeks after the U.S. Food and Drug Administration (FDA) has announced it has fully approved a COVID-19 vaccine or five weeks after Sept. 20, whichever comes first.
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"For those employees who are already vaccinated - and for those employees who get vaccinated and upload their records to Flying Together before September 20th - we'll offer an additional day of pay," United CEO Scott Kirby and United President Brett Hart wrote in a memo to employees.
The executives added, "We know some of you will disagree with this decision to require the vaccine for all United employees. But, we have no greater responsibility to you and your colleagues than to ensure your safety when you're at work, and the facts are crystal clear: everyone is safer when everyone is vaccinated."
United's decision comes as fears mount about the highly-contagious delta variant.
"Over the last 16 months, Scott has sent dozens of condolences letters to the family members of United employees who have died from COVID-19," the executives wrote. We're determined to do everything we can to try to keep another United family from receiving that letter."
Last year, major U.S. airline CEOs pointed to low infection rates among their employees in an attempt to prove that air travel is safe.
"At United, but also at our large competitors, our flight attendants have lower COVID infection rates than the general population, which is one of multiple data points that speaks to the safety on board airplanes," Kirby said during a Politico event in September.
At the time, the largest flight attendant union in the U.S. that represents United flight attendants among other airlines -- the Association of Flight Attendants-CWA, AFL-CIO -- said they saw a little over 1,000 flight attendants across the industry contract the novel coronavirus. That represented less than 1% of the roughly 120,000 flight attendants that were employed at the end of last year, and was lower than the reported general infection rate of 2%.
The video in the media player above was used in a previous report.