“We apologize to our customers for the inconvenience, and the majority have been rebooked for the same travel day,” the airline said in a statement Sunday.
Some employees were having adverse side effects from their COVID-19 vaccinations.
“Delta teams have been working through various factors, including staffing, large numbers of employee vaccinations and pilots returning to active status,” the airline added in the statement.
Air travel in the United States is recovering from pandemic lows. More than 1 million travelers have gone through U.S. airports for each of the last 20 days, although March traffic remains down nearly half from the same periodin 2019.
The numbers are rising heading into the crucial summer vacation season. Last summer was a catastrophe for the airlines, contributing to Delta’s full-year loss of more than $12 billion. The airlines are eager to boost revenue as quickly as possible, and that means selling more seats.
Delta said it had over 1 million passengers during the past few days, the highest number since before the coronavirus pandemic began last year.
The airline took steps to increase passenger capacity, including opening middle seats on Sunday and Monday to accommodate passengers.
On Wednesday, the airline announced that it would stop blocking off middle seats starting in May.
The move was made last April to keep passengers farther apart, a policy that Delta’s CEO had repeatedly cited as raising trust in the airline. The seats would be reopened as air travel recovers and more people become vaccinated against COVID-19, the airline said.
Delta said the middle seats were opened just for Sunday and Monday, and its seat-blocking policy has not changed.
Where needed, seats could be unblocked to get customers to their destinations on the same day.
The airline industry was divided over the utility of blocking middle seats to reduce the risk of spreading COVID-19 on a flight. Airlines including Delta, Southwest, Alaska and JetBlue limited seating for months, while United Airlines never did and American did so only briefly.
Social distancing is hard, if not impossible, on an airplane, even with middle seats empty — a point that United CEO Scott Kirby made many times to explain his airline’s resistance to seat-blocking.
The Associated Press contributed to this report.