Delta Air Lines plans to place 400 pilots back to full flight duties this summer in a sign that the carrier expects the number of passengers to increase over the peak vacation months from current, low coronavirus crisis levels.

The plan is "well ahead" of when the airliner initially projected it would return pilots to regular flying status, Delta Air senior vice president of operations John Laughter said in a Jan. 21 memo.

Delta's aviators avoided forced leave last year after the union struck an agreement to lower salaries and approve no flying requirements for some 1,700 junior pilots.

"We are cautiously optimistic that travel demand will increase as immunizations roll out across the world, and we look forward to restoring all affected pilots back to full flying status as the recovery continues," Laughter said in remarks quoted by CNA.

Union representative Chris Riggins said the Air Line Pilots Association was encouraged by Delta Air's decision.

Laughter's upbeat comments have sparked hopes that a recovery will start after a weak first quarter in which Delta expects to lose as much as $15 million a day.

Shares of Delta fell 2%, and other airline stocks were down even more during afternoon trading as countries tightened travel restrictions to contain the spread of new COVID-19 strains.

Passenger traffic in the U.S. so far this month fell 61% from a year ago. Delta and United Airlines estimate continued sluggish travel demand through March, but airline executives forecast an improvement as more passengers receive vaccines against COVID-19.

Because of pilot and aircraft retirements and reduced flying in the face of a global health crisis, some pilots will require training on new aircraft models, a process that can take months.

The pilots are already being paid regular wages under $15 billion in additional government aid that the U.S. House of Representatives approved late last year in the latest pandemic stimulus boost.