The airline worked with advisers from the Mayo Clinic to establish a protocol that involves multiple tests: a PCR test five days before travel, a negative rapid test at the Atlanta airport prior to boarding, and a second PCR test upon landing in Europe.
"By implementing a testing protocol in the U.S. for these COVID-free flights we're able to make sure that these customers who are traveling are negative and they don't pose a risk to society and their destination city," Delta's senior vice president of alliances and international, Perry Cantarutti, told ABC News.
Passengers boarded Delta's first quarantine-free flight from Atlanta to Amsterdam on Tuesday, and the first flight to Rome is set to take off this weekend.
International travel among U.S. carriers is still down around 70% compared to last year, according to Airlines for America as many countries' borders remain closed to U.S. citizens.
Transatlantic flights are some of the most lucrative routes and airlines have been pressuring governments for months to come to a global agreement on pre-flight testing protocols that could replace restrictive quarantine measures and boost passenger confidence.
"There is a lot of demand out there," Cantarutti said. "People want to travel and we think that being able to avoid quarantine is an enabler that will bring more people to the market."
United Airlines said they saw passenger loads double after the airline established a testing protocol within the U.S. that allowed passengers traveling from San Francisco to Hawaii to bypass quarantine restrictions.
Air travel isn't projected to return to pre-pandemic levels until 2024 -- even with the record-breaking development of the Pfizer and Moderna vaccines.
"The good news is we have the vaccine on the horizon," Cantarutti said. "But we will have passengers who some have the vaccine, some who maybe have immunity from having had COVID, others who have been through testing. We're going to need to be able to accommodate all of those kinds of travelers in order to reopen international travel."
The International Air Transport Association designed a mobile app they hope can serve as a digital health passport where travelers can store verified test or vaccination results. They unveiled design elements of their 'Travel Pass' on Wednesday.
“Testing is the immediate solution to safely reopen borders and reconnect people," IATA CEO Alexandre de Juniac said in a release. "And eventually this is likely to transition to vaccination requirements. In either case, a secure system to manage COVID-19 testing or vaccination information is critical."