ATLANTA -- In August, United Airlines became the first major U.S. carrier to require that employees get vaccinated against COVID-19.

Other major U.S. airlines, including American and Southwest, announced over the weekend and on Monday that they also are requiring employees to get shots. The moves came after Reuters reported Friday that the White House has pushed carriers to do so.

That leaves Atlanta-based Delta Air Lines as the only big U.S. carrier without a broad employee vaccine mandate.

Earlier this year, Delta stood out for a different reason as the only major airline still blocking middle seats. It maintained that policy through April 2021, saying it gained more customers because it made travelers feel safer during the pandemic.

Delta is requiring new hires to be vaccinated. But for existing employees it is instead penalizing unvaccinated workers with a monthly $200 health insurance surcharge starting in November, along with requiring them to take weekly COVID-19 tests and wear masks.

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On Monday afternoon, Delta said its “approach to encourage a high rate of employee vaccinations continues to work,” adding that 84% of its workforce has been vaccinated so far, and that rate is “climbing daily.”

At an International Air Transport Association conference in Boston, Delta CEO Ed Bastian said Sunday, “I’m really pleased with the success that we’re having,” according to a transcript of the remarks.

Chicago-based United said on Monday that 99.5% of its employees had been vaccinated against COVID-19. At the same time, some employees have sued United, which said 320 workers were in the process of being terminated from a workforce of 67,000.

Some industry watchers say Delta may have no choice but to adopt a similar policy, pointing to the Biden administration’s vaccine requirement for federal contractors by Dec. 8.

In a statement Monday, Delta said “we continue to examine the Administration’s executive order.” Bastian said Sunday he expects Delta’s employee vaccination rate to get to “well over 90%” but that the company “will certainly follow any executive order requirements.”

Delta is a federal government contractor. It is one of the U.S. airlines contracted to provide discounted flights to federal government travelers. It is a maintenance contractor for U.S. military aircraft. As part of the Department of Defense’s Civil Reserve Air Fleet, it recently helped transport evacuees from Afghanistan.

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