New York CNN Business  — 

The dreaded middle seat is coming back.

US airlines had stopped selling them for months, both to ensure social distancing and because of a lack of passengers. But with a modest rebound in flying, that is changing.

American Airlines (AAL), the world’s largest carrier disclosed Friday that “customers may notice that flights are booked to capacity starting July 1.” This change comes even as the number of Covid-19 cases rises in many states.

United (UAL) has been willing to sell every possible seat throughout the pandemic. Both said they would notify passengers when a flight has more than 70% of its seats booked, and allow them to change to a less crowded flight. But that won’t necessarily allow passengers with limited flexibility to avoid crowded flights.

The empty seats had been a result of low demand for air travel combined with airline policy meant to encourage people to feel safe about flying. But on Sunday there were 634,000 people passing through TSA checkpoints at US airports, which was 24% of the traffic on the same day last summer. That’s the highest total since late March and is seven times as many people as were screened the low point in mid-April.

And with so many planes parked and fewer flights taking off than a year ago, passengers are quickly filing up a much greater percentage of available seats – about 55% of all seats on planes during the week ending June 21, according to Airlines for America. And that’s been climbing steadily since April.

Delta (DAL), Southwest (LUV) and JetBlue (JBLU) all have policies to leave the middle seat empty. But how long they will stay with those policies is unclear. JetBlue (JBLU) has committed to the empty seats through the end of July. Southwest (LUV) and Delta (DAL) have said the policy will be in effect through Sept. 30.

Delta CEO Ed Bastian told shareholders at the annual meeting two weeks ago that it could lift that limitation come fall.

“And as the business starts to return, as demand starts to grow, and if people have more confidence in their travel experience, we will decide later in this year when we start to ease up on that cap restriction,” he said. But last week he clarified to the BBC that doesn’t necessarily mean it will start selling all seats on the plane starting in October.

“Whether it’s 60% [of seats being sold] or a slightly higher number I don’t know, but yes we absolutely will” keep some limits on the percentage of seats sold in place, he said.

Southwest has thus far not offered any similar assurances past Sept. 30.

The airlines say they are taking other steps to make flying safe, including enhanced cleaning, strict policies requiring the use of masks and filters that remove very small particles including viruses and bacteria from the cabin air.

It’s not clear whether airlines will lose reluctant passengers by deciding to sell the middle seat said Philip Baggaley, chief airline credit analyst for Standard & Poor’s.

“At the moment, most people don’t want to get on an airplane anyway,” he said. “Those that do have made a determination that they have to fly, or feel safe flying. Not sure how large the group is that would be dissuaded by a full plane.”

“But if the recovery in flying continues, there is a greater risk that you are scaring away a more material number of passengers by selling middle seats,” he added.

Airlines elsewhere in the world have not put policies in place that have left seats empty, said John Grant, aviation analyst with tracking service OAG. He said the only broad limit is a rule put in place by Chinese authorities that international flights to and from that country can sell no more than 75% of its seats.

The problem for the airlines is that during normal times they need to sell somewhere between 60% and 73% of seats just to break even, Grant said, depending on their cost structure. US and European airlines typically have the higher break-even point.

Last year US airlines sold a record 85% of domestic seats, helping them to one of the most profitable years on record. But it is not expected to get back to that level of demand for years to come.

Grant says that given seat widths, leaving the middle seat open leaves less than two feet between passengers, not enough for those who want to stay six feet away.

“Keeping the seat open might reassure some passengers, but there is no scientific evidence that you elevate the chance of infection by a factor of Y% by using the middle seat,” he said. “I think there’s less chance of scaring people away because you sell the middle seats than if you charge 30% more to keep the seat empty.”

But others say the airlines do risk scaring away customers who were on the fence about flying.

“People didn’t like middle seats before and they like them even less now,” said Stephen Beck, managing partner of cg42, a management consultant who has advised airlines. “While this will allow them to fill a few more flights to capacity, but will the inevitable negative PR be worth filling those seats? I would question that.”

– CNN’s Gregory Wallace contributed to this report