In the early days of the coronavirus pandemic, even as nations shut their borders and airlines struggled with record-low passenger levels, there was a lot of optimism about “travel bubbles” — a controlled return of quarantine-free air travel between designated cities or countries. Since then, with few countries’ outbreaks truly under control, there has been far more chatter about potential travel bubbles than there have been actual bubbles implemented.

But this weekend, Asia’s first bubble, between Hong Kong and Singapore, will finally make its debut.

The two cities’ “Air Travel Bubble,” set to start Sunday, will test whether regions can safely partner in a return to quarantine-free travel in the pandemic era. The practice could soon emerge in other places, including North America, as scientists learn more about the coronavirus and as nations inch closer to offering vaccines.

So: How do bubbles work? Here’s what you need to know.

What is a travel bubble?

Sometimes called a travel corridor, a travel bubble is a partnership between two or more places with similar rates of covid-19 that allows for quarantine-free leisure travel in both directions.

The first large international travel bubble to make headlines was a potential agreement between Australia and New Zealand, both of which had very low coronavirus caseloads early in the pandemic. The two nations hoped to implement a bubble in September, but those talks sputtered when Australia saw a rise in cases in August. While travel from New Zealand to Australia may not require quarantining, New Zealand still has strict quarantine requirements in place for all arrivals.

Where do travel bubbles exist?

The Baltic countries of Estonia, Latvia and Lithuania created Europe’s first quarantine-free bubble in May. By July, however, the European Union’s “Re-open EU” initiative had rendered it redundant.

You could, in theory, call Re-open EU — which allowed for controlled travel within the border-free Schengen Area and Britain — a travel bubble, although the E.U. and Britain did not. In that agreement, nations were allowed to set their own restrictions and pace beginning in July, but rising coronavirus cases curbed free travel again not much later: England, for instance, recalled quarantine-free travel conditions with Spain two weeks after allowing travel there. Many E.U. nations implemented new restrictions as coronavirus flare-ups emerged, and since October, many nations have again implemented shutdowns or travel limitations, with quarantines and testing required. (Estonia, Latvia and Lithuania have since reestablished their own bubble.)

Hong Kong and Singapore, by contrast, have significantly slowed their outbreaks: Hong Kong has had fewer than 5,500 coronavirus cases, while Singapore has seen about 58,000, with the lowest death rate in the world. When the cities’ bilateral air travel bubble opens Sunday, residents will be able to take advantage of daily flights on Cathay Pacific and Singapore Airlines.

How does a bubble work?

The creation of a travel bubble does not mean that you’ll be able to visit as freely as you would have pre-pandemic. Instead, travel bubbles typically come with testing requirements that must be met to avoid quarantine.

Traveling among Estonia, Latvia and Lithuania, for example, requires a negative coronavirus test result “not older than 48 hours” for entry. The governments of Singapore and Hong Kong, meanwhile, announced that their bubble will require a coronavirus test taken 72 hours prior to arrival and will not permit visitors to have left their country of origin in the previous 14 days.

“Hong Kong and Singapore are similar in terms of epidemic control,” Hong Kong Secretary of Commerce Edward Yau said in a news release. “We hope that aviation, tourism, hotel, retail and catering businesses can benefit from it, thereby enabling Hong Kong’s economy to recover gradually."

Rachel Loh, Americas Regional Director at the Singapore Tourism Board, said in an email that the program will not exceed a quota of 200 travelers per day into each city.

The two cities’ bubble is a potential “model for future collaboration with other parts of the world,” Loh said.

Where could travel bubbles pop up next?

New Zealand Prime Minister Jacinda Ardern, who was reelected earlier this month in part because of her pandemic leadership, said publicly this week that there is little chance a travel bubble with Australia will materialize this year because of ongoing Australian outbreaks.

“What this underscores is why it’s so important that New Zealand has not rushed into this,” Ardern said Monday, according to the Guardian.

But some destinations are still hopeful: New Yorkers buzzed about the potential of reopened air routes to London, a major business-partner hub. In October, the Wall Street Journal reported that the two governments would implement testing between their cities in an effort to open a corridor by the holidays.

That, however, was before Britain recently shut down nonessential travel with a month-long lockdown. But United Airlines is still conducting a trial program of rapid tests between the two cities from Nov. 16 through Dec. 11, calling the initiative “the first free transatlantic COVID-19 testing pilot program.”

“United will share customer feedback of this pilot with governments on both sides of the Atlantic to further demonstrate the effectiveness of these programs as an alternative to mandatory quarantines or duplicative travel restrictions,” Toby Enqvist, chief customer officer for United, said in a statement.

“We believe the ability to provide fast, same-day COVID-19 testing will play a vital role in safely reopening travel around the world and navigating quarantines and travel restrictions, particularly to key international destinations like London."

Traveling during the pandemic: