Delta Air Lines Inc. said it canceled about 100 flights due to a staffing shortage, and as a result will allow travelers to book middle seats on some planes a month ahead of plan.

“Delta teams have been working through various factors, including staffing, large numbers of employee vaccinations, and pilots returning to active status,” a spokeswoman said in a statement on Sunday. “We apologize to our customers for the inconvenience and the majority have been rebooked for the same travel day.”

Delta passengers can also select middle seats for flights Sunday and Monday. The carrier said Wednesday it would resume selling middle seats on May 1, ending the coronavirus-related social-distancing policy after more than a year. With rivals American Airlines Group Inc. and United Airlines Holdings Inc. saying their planes are now flying as much as 80% full, Delta will become the last major U.S. carrier to resume selling all of its seats.

As the pandemic stymied U.S. air travel for much of 2020, Delta cut personnel expenses, contributing to flight cancellations. About 17,000 of its workers left voluntarily and another 40,000 took unpaid leave. The Atlanta-based company posted a loss of $12.4 billion last year.

The rebound in air travel with the acceleration of coronavirus vaccines has put pressure on airlines that downsized. The Centers for Disease Control and Prevention said last week that vaccinated individuals don’t need a Covid test and don’t need to quarantine when traveling domestically.

More than a million customers have flown Delta over the past few days, reaching levels the company hasn’t seen since before the pandemic.