The carrier announced in August that all workers, barring those eligible for a medical or religious exemption, would need to get fully vaccinated by 27 September.
Anyone who refuses will be placed on temporary leave from 2 October.
The six plaintiffs have all refused the vaccine on religious or medical grounds, with four asking for exemption due to their belief that the vaccine was “derived using aborted foetal tissue”.
“United’s actions have left Plaintiffs with the impossible choice of either taking the Covid-19 vaccine, at the expense of their religious beliefs and their health, or losing their livelihoods,” says the lawsuit, reports the Fort Worth Star-Telegram.
The plaintiffs are captain David Sambrano, aircraft technician David Castillo, station operations representative Kimberly Hamilton, customer service representative Debra Jennefer Thal Jonas, flight attendant Genise Kincannon, and captain Seth Turnbough.
Jonas and Turnbough both declined the vaccine for medical reasons; the former due to an allergy to eggs and penicillin and a previous Covid infection and the latter because he has multiple sclerosis, saying his neurologist advised him against getting vaccinated.
The class action complaint was filed on Tuesday at the US District Court for the Northern District of Texas-Fort Worth Division.
United is one of numerous airlines that have made vaccination mandatory for staff.
British airlines have been more cautious when it comes to implementing such measures.
Speaking at an Airlines UK event in February, Shai Weiss, chief executive of Virgin Atlantic, said: “My personal view is that the individual’s right to govern their own body is essential.
“We will do everything we can to promote [vaccination], and explain the merits of being vaccinated for yourself and for your loved ones and for society as a whole.
“But before we take that extra step and say ‘If you don’t have a jab you don’t have a job,’ I think that’s a step too far.”
Steve Heapy, chief executive of Jet2, said: “I agree, and I think that may be subject to a number of legal challenges as well.”