(NEXSTAR) —  An increasing number of airlines are beginning to require crew members to be fully vaccinated against COVID-19, but plenty of major carriers have so far refrained from making it mandatory.

United Airlines issued a vaccine requirement for employees on Aug. 6, becoming the first major carrier to implement such a directive for its U.S.-based staff. Frontier Airlines followed suit later the same day, announcing a similar policy for all “direct” employees. The following week, Hawaiian Airlines issued a memo announcing its own vaccine mandates for the safety of employees and, in turn, its guests.

United is currently giving its employees until Oct. 25 (or five weeks after the FDA fully approves a vaccine, whichever comes first) to comply. Frontier and Hawaiian, meanwhile, have set their deadlines for Oct. 1 and Nov. 1, respectively.

But that still leaves several major carriers — namely, Alaska, American, Delta, JetBlue, Southwest and Spirit — with no vaccination requirements for their employees. Many of the carriers, however, are encouraging or incentivizing the shots.

Here’s what each airline is requiring, or requesting, of employees:

Alaska Airlines

According to an official statement, Alaska Airlines is considering a possible vaccination requirement for employees.

“Due to the highly contagious COVID-19 Delta variant, we are looking closely at whether we will require that employees are vaccinated,” reads an official statement from the airline. “If we do, the requirement would not be effective until at least one vaccine is fully approved by the FDA and would include appropriate religious and medical exemptions.”

While no official mandate has been announced, Alaska Airlines is “strongly” encouraging its staff members to get fully vaccinated against COVID-19 at this time.

American Airlines

American Airlines, too, is “strongly encouraging” its team members to get vaccinated. The carrier is also offering an additional day of paid time off in 2022 and $50 in recognition points for those that do.

Doug Parker, the CEO of American Airlines, recently told The New York Times that he believes this “is the right way to motivate people to get vaccinated.”

“So that’s how we intend to do this,” Parker said in an interview with Kara Swisher published Aug. 5. “We certainly encourage it everywhere we can, encourage it for our customers and our employees, but we’re not putting mandates in place.”

Delta Air Lines

In mid-May, Delta Air Lines announced that all new hires (as of May 17, 2021) are required to be fully vaccinated for COVID-19 unless they qualify for exemption. At the time, Delta said it would not be implementing a “company-wide mandate” for current employees.

“Our people, over 73% of our staff are fully vaccinated and that number is growing by the day,” Bastian added in an August appearance on CNBC’s “Squawk Box,” during which he also said he’s encouraging employees and customers alike to get the shot.

Frontier Airlines

In response to an increase of COVID-19 cases and the spread of the delta variant, Frontier Airlines announced on Aug. 6 that all of its “direct” employees would need to be fully vaccinated as of Oct. 1.

“Safety is of the utmost importance at Frontier and we need to take every step possible for us to keep our teams safe, protect the operation and protect our passengers,” said CEO Barry Biffle in a press release. “The time has come to do what we can to help put an end to COVID-19.”

Frontier, however, is allowing exempt employees (or simply those who “choose not” to get vaccinated) the option of submitting negative test results “on a regular basis,” according to the announcement.

Hawaiian Airlines

Hawaiian Airlines is requiring its U.S. employees to be fully vaccinated against COVID-19 as of Nov. 1, according to an internal memo obtained by KHON2.

“There is no greater demonstration of our values than ensuring the safety of others,” the memo stated. “Safety is the foundation of air travel, and it is ingrained throughout our operation and service. This is no different. By getting vaccinated, we protect ourselves and those around us. That is mālama.”

The airline will also allow employees with medical or religious exemptions to submit to regular testing instead, KHON2 reported.

JetBlue

According to JetBlue, the majority of its crew members “have received the vaccine,” though it is not currently a requirement.

“We highly encourage all crewmembers to get vaccinated for COVID-19, and have set up vaccination sites at some of our support centers and airports,” reads a message posted to the airline’s Safety portal.

Southwest Airlines

Southwest Airlines is also “strongly encouraging” employees to get vaccinated, but does not currently have a vaccine mandate for employees.

In an earlier internal memo obtained by CNN, Southwest CEO Gary Kelly admitted he was “very concerned about the latest delta variant, and the effect on the health and safety of our employees and our operation” but stopped short of making vaccinations a requirement.

Spirit Airlines

Spirit Airlines does not require its employees to be fully vaccinated. Like many other carriers, however, its employees are urged to do so.

In late July, Spirit CEO Ted Christie told CNBC that the airline had considered the possibility of making vaccines mandatory for employees but decided instead to “strongly encourage” them to take that step themselves.

United Airlines

United Airlines is requiring its U.S. employees to be fully vaccinated by Oct. 25 at the latest, but likely sooner if the FDA announces its full approval of a COVID-19 vaccine. In the latter case, employees will have five weeks from the date of the FDA’s announcement to get their shots.

Those who comply earlier, specifically by Sept. 20, will be offered an extra day of pay. Employees who were already vaccinated prior to United’s new mandate will also be extended the same perk.

“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,” reads a letter from United CEO Scott Kirby and United President Brett J. Hart sent to employees last week.

United, too, is preparing to accommodate those who cannot be vaccinated for medical or religious reasons, a spokesperson confirmed.