The message from Delta comes after American Airlines said last week it would start booking flights to full capacity, in line with United Airlines' practice.
Centers for Disease Control and Prevention Director Dr. Robert Redfield said at a Senate hearing earlier this week "there was substantial disappointment" with American Airlines' decision, adding, "We don't think it's the right message."
A United Airlines spokesman called blocking middle seats "a PR strategy," and "not a safety strategy," CNBC reported.
Airlines have said blocking middle seats is a temporary policy and that they could not make a profit over the long term with such a policy.
"As demand starts to grow and as people have more confidence in the travel experience, we will decide later this year when we start to ease up on that cap restriction," Bastian said last month.
Some have also pointed out that blocking a middle seat does not necessarily allow six feet between passengers.
But on Thursday, Bastian wrote in his memo that in addition to the advice from medical experts, “customers tell us it gives them peace of mind when they fly.” The airline has said it would add flights to keep passenger loads under the cap.
A new federal policy document released Thursday by U.S. Transportation Secretary Elaine Chao says airlines “should consider the feasibility of limiting seat availability to enable passengers to maintain social distance from each other during the flight.”
The pilots union at American Airlines last week proposed having the government buy seats on each flight to allow social distancing. Such a policy would encourage passengers to fly more, encourage airlines to operate more flights and preserve transportation infrastructure and jobs, the union said.
Allied Pilots Association President Eric Ferguson said in a written statement that “in order for air travel to return, passengers must feel comfortable that it is safe.”