The U.K.’s aviation industry strongly opposed the new regulation, saying it would financially deliver another blow to crippled airlines by further discouraging travel and tourism.

The U.K.’s British Airways and Easyjet and Ireland’s Ryanair sent a letter to the British government on Friday, criticizing the quarantine as “disproportionate” and “devastating” for the country’s tourism industry, and urged the government not to go through with the measure. Their pleas didn’t stop the U.K. from enacting the measures on Monday.

The U.K. joins several other countries, including Canada, China, and South Korea, in mandating a 14-day quarantine period for arrivals to curb the spread of the coronavirus. The U.S. Centers for Disease Control and Prevention recommends that Americans returning from abroad stay home and avoid social contact for 14 days, too.

For infectious disease specialists worldwide, 14 days is a practical estimate of coronavirus infection risk so there’s near-universal agreement that quarantining for that period can help minimize the spread of the disease. Airlines and other travel industry companies, meanwhile, argue that it disincentivizes travel and thus threatens their business and the larger economy. But so far few reliable alternatives to the two-week quarantine have been found.

In late February, when China was still the epicenter of the coronavirus outbreak, the World Health Organization (WHO) and China conducted a joint mission to learn more about the characteristics of the coronavirus and come up with containment strategies that could be internationally implemented.

The WHO-China report concluded that the mean incubation period for COVID-19 was five to six days, but the full range was one to 14 days, meaning as many as 14 days could pass between when a person was exposed to the virus and the onset of symptoms.

The 14-day figure has been adopted globally; all the countries that mandate quarantine upon entry require it for 14 days.

Self-isolation was and remains a recommended tactic for people who may have the disease but haven’t been tested for it. Worldwide shortages of testing kits and issues with reliability mean quarantining is the safest bet for people who might’ve been infected but can’t get a test.

Since most infected people start showing symptoms in less than a week, the 14-day quarantine period is playing it safe, ensuring that statistical outliers don’t slip through the cracks. There are also asymptomatic people who never exhibit symptoms of the disease; the isolation period decreases their chances of passing the virus onto others who could get sick.

A May 5 study by Johns Hopkins University researchers found that 97.5% of people who develop COVID-19 symptoms will do so within 11.5 days after they are exposed to the virus. This means developing symptoms at 14 days is relatively rare, but not impossible.

“If you want to ensure virus containment, you need a quarantine the length of the maximum incubation period,” said Merle Böhmer, an infectious disease researcher at the Bavarian Health and Food Safety Authority, part of the public health department of Bavaria, Germany.

The shorter the quarantine period, the higher the chance of “virus import,” said Jantien Backer, a researcher in infectious disease modeling at the Centre for Infectious Disease Control at the National Institute for Public Health and the Environment in the Netherlands.

Backer co-authored a study looking at the incubation period of the virus for travelers leaving Wuhan, China—where the first coronavirus outbreak occurred—in one week in January. They found that 5% of the 88 cases included in the study had incubation periods longer than 11 days.

“Quarantining is a very important tool to be able to control COVID-19,” Backer said. “The 14-day quarantine period is a prudent choice to capture most of the infections in this time period.”

There is a broad scientific consensus that 14 days is an acceptable estimate of incubation time, but the travel and aviation industries—hard hit by the economic fallout of the pandemic—are not convinced that two weeks of isolation is necessary.

“We are urging governments to find alternatives to arrival quarantine,” said Albert Tjoeng, Asia-Pacific assistant director of corporate communications for the International Air Transport Association (IATA), a trade group representing 290 airlines.

An April IATA survey of travelers found that 69% of people would not consider traveling if it involved a 14-day quarantine period, and 86% were “somewhat or very concerned” about being quarantined while traveling.

“Few businesses will be willing to have staff travel if it means 14 days of quarantine on both ends of the trip, and staff may be unwilling to make that same sacrifice,” said Todd Handcock, Asia-Pacific president for Collinson Group, a customer benefits and loyalty firm that works globally with airlines, airports, and travel companies.

IATA’s alternative suggestions include temperature screening to find symptomatic passengers and stop them from flying, and health declarations and contact tracing on arrival to reduce the risk of imported cases.

Tjoeng said with these guidelines, “quarantine measures can be eliminated,” and said the guidelines are a “temporary” approach to be used “until a vaccine, immunity passports, or nearly instant COVID-19 testing is available at scale.”

In a bid to attract much-needed tourism revenue, Iceland is offering visitors the option to skip the mandatory 14-day quarantine if they pay $115 for a coronavirus test.

And since early May, the Vienna International Airport has offered coronavirus tests too, for people who reside in Austria. They pay $208 and get results in three to six hours; if they test negative, they can bypass the otherwise mandatory 14-day quarantine. Non-residents can skip quarantine if they present a medical certificate proving they’ve tested negative for coronavirus in the last four days.

“If [Vienna’s] approach could be more broadly adopted [and] supported by regulated guidelines, this could be a way to remove the need for prolonged implementation of quarantine periods,” Handcock said.

Other places are being even more cautious. Passengers arriving in Hong Kong pass a temperature check before they reach immigration, and then take a bus to a facility to take a coronavirus test and await the result. Even if they test negative, they are required to self-isolate for 14 days, and all arrivals are fitted with an electronic location-tracking bracelet to ensure they don’t break quarantine. The measures seem to be paying off—Hong Kong has kept new local cases low and only four people have from coronavirus died there.

Testing also helps detect asymptomatic cases, which a quarantine with no test would not. But coronavirus testing for all arrivals isn’t foolproof. If a person has low concentrations of the virus in their body—concentration is low at the beginning of infection—then a polymerase chain reaction (PCR), the standard coronavirus test, might not be able to detect it.

For this reason, exchanging a longer quarantine period for a PCR test and a shortened quarantine might be ill-advised.

“In my opinion, testing on arrival combined with a shorter quarantine period will not help in decreasing the risk of virus import,” Backer said.

Correction: This story has been updated to correct the spelling of Todd Handcock’s name.