Traveling between the islands in Hawaii by commercial flights or chartered aircraft is easy, but it can take more time than travelers expect.

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Hawaiian Airlines told U.S. employees on Monday that they needed to be vaccinated against Covid-19, making it the third major airline to issue such a mandate in less than a week.

CEO Peter Ingram told employees that if they receive a two-dose vaccine, they must have their second vaccination by Nov. 1, although there will be exceptions for medical or religious reasons, according to a CNBC-verified employee note.

Last week, United Airlines became the first major airline in the country to mandate vaccines, requiring their 67,000 U.S. employees to provide proof of vaccination by October 25th at the latest. Frontier Airlines also announced that their employees must be vaccinated against Covid or regularly tested by October 1.

“There is no better demonstration of our values ‚Äč‚Äčthan ensuring the safety of others,” Ingram said. “Safety is the foundation of air travel and it is ingrained throughout our operations and service. It is no different.”

More than a dozen large U.S. companies have issued vaccination mandates to all or some of their employees in the past few weeks, with some executives citing concerns about the rapidly spreading Delta variant of Covid.