Two new Covid-19 testing programs may be the key to unlocking travel between the United States and Europe.

Select passengers flying on American Airlines, British Airways and United Airlines flights are eligible to participate in testing programs that are evaluating whether preflight Covid-19 tests can be used to relax travel restrictions imposed by both countries.

The two programs, however, are quite different. Here is what to know about each.

American Airlines and British Airways

On Nov. 17, American Airlines and British Airways announced the launch of an optional Covid-19 testing trial on flights from the U.S. to London.

The testing program will begin on Nov. 25 on flights to Heathrow Airport departing from Dallas Fort Worth International Airport (Flight AA50), New York's John F. Kennedy International Airport (BA114) and Los Angeles International Airport (BA268). A second flight originating from JFK (AA106) will be added to the program at a future date, according to an American Airlines' press release.

Passengers who volunteer for the program will undergo three free Covid-19 tests as follows:

• Test 1: A self-administered RT-PCR (nasal) test taken at home via a virtual consultation with a medical professional 72 hours prior to departure
• Test 2: A LAMP (nasal) test taken upon landing at Heathrow Airport
• Test 3: A self-administered saliva test taken at home three days after arriving in the U.K. 

The three-test approach is specific to the trial, said David Evans, joint CEO at Collinson, a global travel company that began operating two Covid-19 testing facilities at Heathrow for outbound travelers in mid-October.

The goal "is to capture data to demonstrate the effectiveness of the different tests used at different stages," he said.

"RT-PCR tests are the 'gold standard' … because they are even more sensitive than LAMP tests, but they also take anywhere between six to 24 hours to process," Evans said. "RT-LAMP tests — which are both highly effective and can be processed in around 60 minutes — are ideal for an airport environment as they can be processed on-site."

Even if all tests are negative, passengers will still need to self-isolate in the U.K.

The trial will end after 500 customers have been tested under the three-test approach, said Janine Brown, an American Airlines' communications manager.  

"The trial is being conducted with the aim of showing that testing can play a role in reducing the need for quarantine," Brown told CNBC's Global Traveler. "The U.K. government has also said recently it is evaluating testing as a means to reduce the self-isolation period."

Evans said research shows the U.K. alone is losing 32 million British pounds ($42.3 million) every day due to lack of flight connectivity with the United States.

Before the global pandemic, American Airlines and British Airways flew up to 111 flights a week between New York and London. Now, the carriers are operating 14 weekly flights between the two cities.

Doug Parker, chairman and chief executive of American Airlines, said in a company press release that the airline has already successfully introduced Covid-19 testing to customers traveling from the U.S. to the Caribbean and Latin America. American Airlines currently tests passengers traveling to Grenada, Jamaica, St. Lucia, Belize and Hawaii.

"The U.K. is a critically important business and leisure destination that our customers want to visit," said Parker. "We believe the results provided by this trial will be vital for reopening transatlantic travel safely."

United Airlines' preflight testing program

On Nov. 16, United Airlines began a month-long preflight testing trial on flights from Newark Liberty International Airport to Heathrow Airport (UA14).

Testing is free, however, the similarities between the two programs end there.  

Participation in United's program is mandatory. All passengers over 2 years old, including flight crew members, are required to take a rapid Covid-19 test before boarding. Passengers who do not want to participate will be rebooked on a different flight at no additional cost.

Here, only one test — a rapid Abbott ID Now test — is required. The White House relied on Abbott's ID Now test to screen visitors before an outbreak occurred in September, which culminated in President Donald Trump being diagnosed with Covid-19 on Oct. 2.

The test accurately diagnoses Covid-19 cases in those presenting symptoms, but is known to miss as many as one-third of asymptomatic infections. United Airlines did not respond to CNBC's request for information regarding the efficacy rates of this test.

Testing is done at the airport via an appointment, which the airline advises passengers book at least three hours before their departure times. Results are available approximately 30 minutes later.  

Passengers who test positive cannot board the flight. Pursuant to the airline's terms, these passengers will either be refunded or rebooked on another flight departing at least 10 days later. Those who test positive are required to show two successive negative Covid-19 results to board a rebooked flight.

Additional terms state: "You acknowledge that lab test results are not guaranteed to be accurate, and you may receive a false positive or a false negative finding. You agree that United bears no responsibility for an inaccurate test result and any resulting consequences."

The benefits of testing may outweigh the risks, however. The airline reports a "positive impact on travel demand" and significant increases in revenue when testing options are available, according to a United Airlines' press release issued on Monday.

Currently, most residents of the European Union are prohibited from entering the United States, under a travel ban issued last March. Americans can travel to the U.K. if they quarantine for 14 days upon arrival.