The new normal for air travel in 2021 could include two, three or even more COVID-19 tests per trip until vaccines are widely available.

The big picture: Global travel could begin to see a comeback later in 2021 as people get vaccinated and international borders reopen. But the healthiest people — those most likely to travel — will be vaccinated last. In a partially vaccinated world, passengers will still need to wear masks and get tested before, during and after their journey.

• "For the next two to four years, different people will have different vaccines but not everyone will have a vaccine. What that means is everyone on the flight still has to be tested," says Simpliflying CEO Shashank Nigam.

Driving the news: Delta Air Lines is offering "COVID-free" flights for essential travel between the U.S. and Europe. But passengers must have three negative COVID tests — two before departure and one upon arrival in Amsterdam or Rome — in order to avoid the usual 14-day quarantine.

• The rules are slightly different for each destination, so travelers need to know what kind of test (PCR or antigen) will be accepted and when to get it.
• Travelers returning to Atlanta from Rome will need yet another airport test before the return trip.

But, but, but: Entry requirements are evolving since it's the individual governments — not the airlines — that set the rules depending on the course of the virus.

The good news: Faster, more convenient testing options are emerging.

• This week, the FDA granted emergency authorization to Ellume's over-the-counter antigen COVID-19 test that can produce results at home in about 20 minutes.
• One new PCR test uses a simple mouthwash rinse to detect COVID with 99% accuracy, says its distributor, Simpliflying. The test, which still requires the sample to be overnighted to a lab, is approved for use in Europe and is seeking FDA authorization.

What's next: Vaccines could one day be required for international travel, but there are no uniform requirements across the world, which means getting vaccinated in one country might not guarantee entry into another.

• The U.S. and some western countries have approved Pfizer-BioNTech's vaccine and Moderna's could be next.
• But China and Russia have developed their own vaccines, and some Middle Eastern countries have already approved a vaccine by Sinopharm, a Chinese-state owned pharmaceutical company.
• A total of 61 COVID-19 vaccines are in clinical trials around the world, according to the New York Times' vaccine tracker.

What to watch: Some airlines and airports are starting to roll out CommonPass, a global platform created by the World Economic Forum and Swiss-based non-profit The Commons Project.

• CommonPass allows people to digitally document their COVID-19 status — through test results and eventually, vaccinations — while protecting their health data privacy.
• The system taps its CommonTrust Network of public and private partners to assesses whether the information comes from a certified lab or medical system and then whether it satisfies the health screening requirements of the country they want to enter.
• Validation is provided through a simple digital code but the underlying health information stays private.
• CommonPass is designed to work with other digital health apps from companies like IBM and Clear.
• Yes, but: "It's not a silver bullet that's going to solve everything," Aaron McMillan, United Airlines' managing director, operations policy and support, tells Axios.

The bottom line: COVID-19 testing will be part of the hassle of traveling for the foreseeable future.