To block or not to block. The middle seat has become a critical issue for airlines as coronavirus-wary leisure travelers straggle back onto planes.

Southwest Airlines Co. said Thursday that it would begin filling middle seats on Dec. 1. The change leaves Delta Air Lines Inc. and Alaska Air Group Inc. as the only two major U.S. carriers that will keep those seats off limits into early 2021.

“It is a very safe environment with all of the air-filtering technology and wearing masks,” Southwest Chief Executive Officer Gary Kelly said on CNBC after the carrier reported earnings. “The science supports that.”

The seat shift on its Boeing Co. 737 narrow-body jets will make flight schedules more predictable, said the Dallas-based carrier, which will allow passengers to change flights to avoid sitting between two other travelers.

Airlines worldwide have had to balance passenger fears of contracting the virus with the companies’ own needs to remain solvent as travel plummeted. Even with restrictions loosening up in the U.S., air traffic is only about 35 per cent of what it was a year ago.

Alaska said it would block middle seats until at least Jan. 6. The airline plans to make them available first on short-distance flights and those to Hawaii, which requires visitors to produce a negative coronavirus test to avoid a 14-day quarantine. Delta also has said the spots would be off limits at least through early January.

Alaska will use the next few months to “educate” customers and employees that flying is safe and then alter its policy, President Ben Minicucci said on an earnings call.

The Seattle-based carrier’s decision is partly because leisure traffic has begun to return and middle-seat capacity is required. Moreover, the airline can’t break even on a cash-flow basis with middle seats blocked, executives said.

JetBlue Airways Corp. continues to block “the vast majority of middle seats on our larger aircraft and the majority of aisle seats on smaller aircraft,” spokeswoman Emily Martin said by email.