Delta originally introduced the middle seat block in April 2020. Delta CEO Ed Bastian said after the initial announcement the decision coincides with customers' trust in the airline.
"Our internal research has shown that 64 percent of Delta customers anticipate having at least one dose of a COVID-19 vaccine by May 1, increasing to 75 percent by Memorial Day," Bastian said in memo to employees on March 31.
The Transportation Security Administration said Friday it will extend its mask requirement, which also applies in airports and train stations, through Sept. 13. Delta has a company mask policy and said it will keep its cleanliness standards in place.
Delta temporarily placed passengers in middle seats when the airline experienced staff shortages in April.
Delta reported it was still suffering losses during the first quarter of 2021, linked to reduced capacity.
Delta has pointed to some studies that indicate a low risk of viral transmission on aircraft while passengers wear masks.
The risk of transmission may depend primarily on the length of the flight. U.S. Centers for Disease Control and Prevention published a study estimating that leaving middle seats empty reduces the risk of COVID-19 transmission by up to 57%. Airline industry officials criticized the study for failing to consider face masks and vaccinations.
One study from November 2020 advised that guidelines for preventing SARS-CoV-2 infection on airplanes should consider the flight duration, among other factors, after it found 75% of COVID-positive passengers tested after a 10-hour commercial flight were seated in more spacious business class seats.
The Associated Press contributed to this report.
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