The number of passengers placed on a “no-fly” list under Delta’s strict mask mandate has more than doubled since early September, when the Atlanta-based carrier had reportedly blacklisted 270 travelers.

The total “represents a tiny fraction of our overall customers, the vast majority of whom follow our guidelines and appreciate the steps we are taking to keep them safe and healthy,” Bastian said in a memo to employees stressing the importance of mask-wearing and other coronavirus safety measures.

Major US airlines including Delta, United, American and Southwest first threatened to ban travelers for not wearing masks in June in an effort to protect workers and passengers from COVID-19.

The mask mandates are “perhaps the most essential layer of a comprehensive set of measures to reduce transmission of COVID-19 throughout air travel,” according to a report from Harvard’s T.H. Chan School of Public Health.

However, Bastian said the pandemic will likely weigh on the airline industry into next year despite recent progress toward a vaccine that could help the economy get back to normal.

US airline passenger volumes remain 65 percent lower than they were a year ago as the nation grapples with a record-setting surge in coronavirus cases, according to industry group Airlines for America.

“Keep in mind that while developing a vaccine is an important step, widespread distribution will take many months, so we continue to expect 2021 will be a year with continued challenges,” Bastian wrote.

Delta shares were up about 1.2 percent at $35.42 as of 11:49 a.m. Thursday.