Staffing shortages forced Delta to cancel around 100 flights on Sunday and open up middle seats to handle its passenger load.
The staffing shortage happened over Easter weekend, with View From The Wing noting that similar cancelations happened over Thanksgiving and Christmas.
It came as the airline was experiencing its busiest period of travel since the COVID-19 pandemic began, and the shortage prompted the airline to temporarily abandon its closure of middle seats, a measure it had taken to maintain distancing on flights.
Delta had planned to reopen middle seats at the beginning of May, but made the concession for flights on Sunday and Monday so it could cope with the staff shortage.
Of the flights that were canceled, the Associated Press reports that at least three of them were due to land or depart from Minneapolis-St. Paul International.
"Delta teams have been working through various factors, including staffing, large numbers of employee vaccinations and pilots returning to active status," Delta 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."
There have been growing signs that Americans are returning to travel as the nation's vaccination program has been rolled out.
But cases are on the rise again in many states including Minnesota, which has experienced a resurgence in the virus driven by the more contagious and potentially deadlier B.1.1.7 U.K. variant.
Minnesota posted more than new 2,500 cases of COVID-19 on Saturday, the fourth day in a row it recorded more than 2,000 cases.