One of the strategies for keeping international travel afloat during the pandemic was the creation of “travel bubbles”: alliances between neighboring nations with low infection rates that would allow travelers from those countries to freely visit.

But as the summer draws to a close and infections continue to pop up, it appears that, in many places, the travel bubble has burst.

There was a lot riding on the alliances. The pause on international tourism doesn’t just spoil family vacations; it affects the global economy in profound ways, too. The World Travel and Tourism Council estimates that about one in 10 jobs are related to tourism and travel. For popular destinations like Greece, where the tourism sector is responsible for about 40 percent of jobs, the effect is even more significant.

In May, New Zealand and Australia garnered attention when they announced a plan to create one of the first travel partnerships during the pandemic. The “Trans-Tasman bubble” would allow citizens of each country to travel to the other without a quarantine or a test. The hope was to enact it by early September, but in early August, a coronavirus outbreak in the Australian state of Victoria put those plans on hold.

The risk is high both for travelers and the countries they are visiting, said Dr. Brad Connor, the New York City site director for the GeoSentinel emerging infectious disease surveillance network of the C.D.C. and a longtime travel medicine and infectious disease specialist.

Asymptomatic and pre-symptomatic travelers carrying the virus can inadvertently bring it into an area undetected and cause an outbreak for which there is no vaccine and no reliable treatment, Dr. Connor said. “We all want to travel again in a safe way,” he said. “Unfortunately, the approaches so far have not worked.”

Infection waves can be local and some countries’ restrictions get down to regional specifications. Belgium’s list recommends testing and quarantine for travelers arriving from certain areas within other European Union countries, like the southwest regions of Bulgaria. Norway declared that the region of Norrbotten in Sweden, which has fewer Covid cases than most other areas in the country, will be changed on its list from “red” to “yellow,” and anyone entering Norway from there will not be required to go into quarantine.

Cordoning off an area of the country for international tourists is the latest idea aimed at preserving a portion of this year’s tourism revenues. Thailand is considering a new program where international travelers who are willing to stay for 30 days are allowed into Phuket, where they will quarantine in specified resorts and undergo two Covid-19 tests.

The island of Anguilla in the Caribbean, which said it had no current virus cases, recently announced it would begin welcoming travelers who must apply to visit, giving preference to those from countries with lower infection rates and those who plan an extended stay. Travelers must arrive with evidence of a recent test showing they do not carry the virus. They will also need to quarantine for 10 to 14 days when they arrive, undergo two more virus tests and pay a fee for these services. The web portal states that “Anguilla is open for approved visitors from certain countries.”

Most countries now realize that they can’t plan too far in advance. The government website of Norway advises citizens that it “is not possible for the Ministry of Foreign Affairs to say when the travel advice for countries outside Europe will be changed.”

Travelers can continue to minimize risk to themselves and others with masks, hand-washing and social distancing — “but at this point we can’t completely eliminate the risk,” Dr. Connor said. “And therein lies the problem.”