Although the coronavirus is still proliferating around the globe, airlines have begun to look at what it will take to get fliers back on planes en masse in a post-pandemic world.

Blocking middle seats in the name of social distancing and deep sanitizing of plane cabins will likely be sticking around for the foreseeable future, but another possible preventative step has surfaced in the industry: immunity passports for COVID-19. Delta Air Lines' CEO became the latest person to embrace the potential document on Wednesday.

Speaking on the airline's first quarter earnings call, Delta CEO Ed Bastian noted that the carrier was open to the idea if it helped passengers feel more comfortable on board aircraft again.

"We will make whatever changes to the business model that will be necessary," Bastian said on the call. "If it turns out immunity passports will be a new form... You think about everything that came out of 9/11 with TSA and homeland security and new public agencies. Could there be a new public health agency coming out that requires a new passport to travel? We'll be on the forefront of all those advances."

An immunity passport is essentially a document issued to anyone who has recovered from COVID-19, and therefore whose immune systems would theoretically have the antibodies necessary to fight off the virus and prevent a second infection. States like New York have begun testing citizens for antibodies. Some government officials are playing with the idea of lifting stay-at-home orders for individuals with the proper antibodies.

At least one nation, Chile, has taken steps to issue COVID-19 immunity passports to people who have recovered from the virus. Those who hold the documents are allowed to break lockdown measures and do things like return to work in their communities, but there are still certain restrictions on travel. Other countries, including Italy, Germany, and the U.K., are seriously examining the idea of issuing immunity cards.

There are also reports that U.S. officials could be open to the idea of immunity documentation. Immunity cards could "have some merit under certain circumstances," Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said on CNN in early April. "It's one of those things that we talk about when we want to make sure that we know who the vulnerable people are and not," he said.

Medical researchers, however, do not have definitive evidence that the presence of antibodies guarantees immunity from COVID-19. "We do not know enough about the new coronavirus yet to determine whether survivors are really immune," the Infectious Diseases Society of America said on Twitter. "COVID-19 is such a new disease that there is no solid data on the immunity of survivors."

Likewise, the World Health Organization recently issued a scientific brief that discourages countries from issuing immunity passports. "There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection," the WHO's brief states. In fact, according to the WHO, the documents could actually accelerate spread of the disease. "People who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice. The use of such certificates may therefore increase the risks of continued transmission," the health bulletin says.

There have indeed been instances of significant reinfections. About 2 percent of patients who had recovered from the virus in South Korea, for example, were forced to return to isolation after contracting COVID-19 for a second time, the WHO told Reuters.

Other critics say the cards could lead to job discrimination, a black market for immunity passports, or people intentionally trying to become infected with the coronavirus in order to recover and be allowed return to work.

In lieu of immunity passports, some airlines like Emirates have begun administering rapid result blood tests for the coronavirus before boarding on certain repatriation flights. If a passenger tests positive for COVID-19, they are not allowed to board.

Although the exact safety measures that airlines and government officials will take once mass air travel opens up again aren't yet certain, passengers will clearly have health concerns going forward.

When you ask what's the most important thing to get them traveling again, Bastian said, "it's going to be confidence in their safety, their personal safety, not just their physical safety."

We're reporting on how COVID-19 impacts travel on a daily basis. Find all of our coronavirus coverage and travel resources here.

This story was last published on April 23, 2020. It has since been updated with new information.