With no federal mandates to follow, an increasing number of U.S. airlines and airports are offering preflight coronavirus testing to boost public confidence in flying during the pandemic and help restore their businesses.

The move mirrors what is already being done in countries around the world where preflight testing is seen as a way to reopen for business while helping control the spread of the virus.

More than 100 countries now require proof of a negative coronavirus test for entry, and in some cases travelers with negative results are allowed to skip otherwise mandatory quarantines. The International Civil Aviation Organization, the United Nations body that oversees aviation, issued new recommendations last week that acknowledged the potential of such programs. The organization’s approach leaves the decision regarding quarantines up to individual nations, but it said it would publish a manual in coming days to help governments develop policies.

“It was recognized that introducing testing could — if properly implemented in States that assess it as appropriate for their situation — reduce reliance on measures such as quarantines that restrict air travel or the movement of persons arriving in a country, and which evidence suggests is a disincentive to several important categories of travel,” the U.N. group said in a statement.

Many international airports, including in Turkey and Germany, were among the first to offer such testing. In recent months, airlines and airports in the United States, including in Connecticut, Florida, New York and San Francisco, have begun offering the option as air travel continues to stall.

Domestic air travel is down 64 percent compared with what it was a year ago; international travel lags even more, down 74 percent for the same time period. At the same time, U.S. coronavirus cases are surging, with the seven-day average of new cases on Monday reaching its highest level, topping 150,000. Overseas, some countries have reinstituted shutdowns.

“I talk to a lot of people in the industry right now who say in the absence of a vaccine, they feel that testing is a necessary step to get [people] flying again,” said Kevin A. Dillon, executive director of Bradley International Airport in Hartford County, Conn., which began offering testing at the end of September. Since then, more than 4,000 people have been tested and the airport’s partner, Genesys Diagnostics, has hired additional staff to meet the demand.

The Centers for Disease Control and Prevention continues to caution that even with masks, social distancing and other measures, travel still poses some risk, but health experts say testing could add another layer of protection.

Simply put, “I believe that we need to be integrating efficient and effective testing in large transportation systems,” said Mara Aspinall, a biomedical diagnostics professor at Arizona State University and adviser to the Rockefeller Foundation.

Abraar Karan, a doctor at Brigham and Women’s Hospital and Harvard Medical School, said testing could serve two purposes: reducing the risk of the virus spreading onboard planes and limiting infections between different communities. When the virus is spread widely across the country and the world, catching infected people leaving areas with higher rates of transmission could help slow it down.

But Karan said there are trade-offs. False positives might keep passengers who could actually travel safely off planes. And perhaps most importantly, a single test is just a snapshot in time and won’t detect people in the early stages of the illness. It may take up to 14 days after a person is exposed for symptoms to develop.

“Testing is certainly not a free pass,” Karan said, emphasizing that quarantining remains the surest way of isolating people who might be infectious.

Janet Hamilton, executive director of the Council of State and Territorial Epidemiologists, agreed.

“I understand that [quarantine] is hard,” Hamilton said. “People want to test and say, ‘I’m going to be fine,’ but it is also true that travel itself can be an exposure event.”

There is no common standard, so it has been left to airlines and airports to design their own testing programs and for travelers to sort out requirements for their particular destination.

At least 120 countries require travelers to have received a negative test result before arriving in the country, generally within 72 hours of their flight, according to a tally maintained by Shoreland Travax, a travel medicine database. Another 50 countries allow people to avoid a quarantine or other restrictions if they have tested negative. More than 80 countries require a test upon arrival, sometimes even if a passenger was tested before leaving their home country, according to the database.

In October, United, Hawaiian and Alaska airlines were among those that announced they would offer testing to Hawaii-bound customers. Hawaii has a mandatory 14-day quarantine for arriving travelers, which those who test negative are able to skip. More than a dozen states have quarantine requirements in place, though some, like Hawaii, may waive requirements with evidence of a negative coronavirus test. Passengers who test positive for the virus aren’t permitted to fly. Many airlines also require travelers to fill out health questionnaires indicating whether they have tested positive for the virus or have been in contact with someone who has.

Tampa International Airport launched a voluntary testing program in October that is open to all ticketed passengers. Joe Lopano, the airport’s chief executive, said he views it as a way to help both customers and the airport’s airline partners.

“This could be, especially if adopted by other airports, another way to instill confidence,” he said.

Airport officials said more than 3,000 people were tested during the one-month pilot program that began Oct. 1. The program proved so popular that the airport’s partner, BayCare, had to add staff to keep up with the demand. It will now run through the end of the year, officials said.

It’s clear why the industry is eager to expand the use of testing. Last month, United Airlines said its bookings almost doubled after it launched a partnership with the state of Hawaii that allows customers flying from San Francisco to take coronavirus tests before boarding their flights. In general, travelers pay for the tests, which range in cost from $57 to $250 depending on the program and the type of test.

Daniel Chun, a spokesman for Alaska Airlines, said its testing program proved so popular that by mid-October appointments were booked through early November. He estimated that roughly 85 percent of the airline’s customers opt to be tested before they fly.

Sharon Pinkerton, senior vice president for legislative and regulatory policy at Airlines for America, a trade group that represents most major U.S. carriers, said Hawaii’s use of preflight testing demonstrates that it can be an effective tool for restarting travel while keeping communities safe.

“What the early data shows us is that this program is highly effective, helping ensure safe travel and providing that much-needed surge in demand.” Pinkerton said. “In other words, it’s good for keeping passengers safe and helping fuel economic recovery. It’s not an either-or choice.”

Even so, testing is not foolproof. A study by the state of Hawaii found that there have been a small number of instances where travelers who tested negative for the virus before their flights later tested positive — some shortly after their arrival.

Still, experts say these pilot programs enable officials to experiment with different approaches that may help when countries begin to think about testing as a strategy for reopening more broadly while still controlling the virus.

“We need to be using the time now, when volumes are relatively low, to test the systems and gain insight on which protocols are most effective,” said Aspinall, the Arizona State professor.

As the use of such testing becomes more widespread, so does the potential for fraud. Systems will be needed to ensure that travelers are presenting valid test results. The importance of such safeguards was demonstrated earlier this month when seven people were arrested and charged with selling false test certificates at Paris’s Charles de Gaulle Airport. Authorities said the investigation started after a passenger checked in for flight to Ethiopia in September with a faked document. The group was selling the certificates for 150 to 300 euros ($180 to $360). They face up to five years in prison if convicted.

In October, the Commons Project Foundation, a Swiss nonprofit, carried out a transatlantic test of a digital system for verifying air travelers’ test results that could help prevent such incidents. The organization hopes the tool, which it calls CommonPass, will give governments greater confidence in the validity of test results, replacing paper certificates that airlines and governments in many countries rely on with a mobile app.

As vaccines become available, the system could be used to verify vaccination records, too, officials said. The test was observed by U.S. Customs and Border Protection and the CDC, and involved a group of volunteers who traveled from London Heathrow Airport to Newark Liberty International Airport on a United Airlines flight.

Brad Perkins, chief medical officer for the Commons Project, said testing airline passengers holds great promise, especially if a system could be developed that involves testing before departure and on arrival to account for the virus’s incubation period.

“That will do an excellent job minimizing risk,” said Perkins, a former chief strategy and innovation officer at the CDC.

The CDC was noncommittal on whether CommonPass might be a viable alternative.

In a statement, Marty Cetron, director of the agency’s Division of Global Migration and Quarantine, called the pass “one of the many potential tools that may one day contribute to a safe, responsible and healthy global air travel experience.”

Another key, said Karan, the physician affiliated with Brigham and Women’s Hospital and Harvard Medical School, will be coordination. To be most effective, Karan said, testing passengers would need to be coordinated by the federal government. But federal agencies have not shown any willingness to impose new rules on airlines during the pandemic.

“If they do nothing, we’ll continue to have a completely uncoordinated strategy, which isn’t going to work,” Karan said.

The industry agrees.

“Our ask again is that our federal and state governments work together with us to come up with a harmonized approach to testing and eliminating these travel restrictions,” said Airlines for America’s Pinkerton. “A patchwork not only creates confusion among passengers and I think reduces public confidence, especially as we try to ensure that all of the measures that we’re undertaking are standardized across the system.”

The Trump administration has left many of these such decisions to states, which has made coordination difficult. However, President-elect Joe Biden has said he supports a coordinated strategy for combating the virus and has appointed a coronavirus task force.

“Fundamentally, information is power,” Aspinall said. “The only way to take back control and get our lives back is through information, testing and, when necessary, isolation.”