United Airlines will have to cut some U.S. flights because thousands of employees are getting sick with COVID-19, the carrier's chief executive said Tuesday.

In a letter to employees posted on the United website, CEO Scott Kirby said no vaccinated United employees have died or been hospitalized with COVID-19. However, he said the rate of illnesses is disrupting the carrier's flight schedule.

Kirby said about 3,000 United employees -- about 4% of its U.S. workforce -- are currently sick with COVID-19, and some locations have been hit particularly hard.

"While we go to great lengths to avoid canceling flights, we worked to get ahead of the impact by acting early to cancel flights when necessary and notifying impacted customers in advance of them coming to the airport," Kirby wrote in the letter.

"Just as an example, in one day alone at Newark [New Jersey], nearly one-third of our workforce called out sick."

Kirby didn't mention specific routes or flights that will be canceled, but noted that United's vaccine mandate has kept the staffing situation from becoming even worse.

"Since our vaccine policy went into effect, the hospitalization rate among our employees has been 100 times lower than the general population in the United States," he said.

United and other U.S. carriers canceled thousands of U.S. flights over the holiday travel season due to rising COVID-19 illness among workers.

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