Taking an act-now-ask-questions-later approach, countries around the world are slamming their doors shut again as the World Health Organization warns that the global risk from the omicron variant of the coronavirus is “very high” and could result in future surges with “severe consequences.”

The UN health agency, in a technical brief to member states, said “considerable uncertainties” remain about the new variant that was first detected in southern Africa, but that, based on the possibility that mutations in omicron could escape an immune response and boost its ability to be transmitted between people, “the likelihood of potential further spread of omicron at the global level is high.”

To that end, an increasingly list of countries introduced new travel restrictions designed to keep the variant at bay. Japan announced it would bar entry of all foreign visitors, joining Israel in doing so just days after the variant was identified by researchers in South Africa. Morocco banned all incoming flights. Other countries, including Canada, the US, and European Union members, have moved to prohibit travellers arriving from southern Africa.

Travellers infected with the new version have turned up in a cascading number of countries over the past few days and new cases in Spain, Portugal and Scotland have raised fears that the variant may already be spreading locally. On Sunday, Canada reported that two cases had been identified in Ottawa and another was flagged in Quebec on Monday.

“Many of us might think we are done with COVID-19. It’s not done with us,” warned Tedros Adhanom Ghebreyesus, director-general of the World Health Organization.

The infections showed the near impossibility of keeping the genie in the bottle in a globalized world of travel and open borders.

Yet, many tried to do just that, even against the urging of the WHO, which noted that border closings often have limited effect and can wreak havoc on lives and livelihoods.

Some argued that such restrictions could provide valuable time to analyze the new variant. Little is known about it, including whether it is more contagious, more likely to cause serious illness or more able to evade the protection of vaccines.

While the initial global response to COVID-19 was criticized as slow and haphazard, the reaction to the new variant came quickly.

“This time the world showed it is learning,” said EU Commission President Ursula von der Leyen, singling out South African President Cyril Ramaphosa for praise. “South Africa’s analytic work and transparency and sharing its results was indispensable in allowing a swift global response. It no doubt saved many lives.”

The WHO has praised Botswana as well as South Africa for quickly alerting the world to the presence of the new variant — and many have warned the countries should not be punished for their speed.

But that did not hold von der Leyen back from successfully pushing the 27-nation EU to agree on a ban on flights from seven southern African nations over the weekend — similar to measures many other countries have taken. On Monday, EU members Spain and Poland followed through by announcing travel and quarantine restrictions.

Cases had already been reported in EU nations Belgium, Denmark, and the Netherlands before Portuguese authorities identified 13 cases of omicron among team members of the Belenenses professional soccer club. Authorities reported that one member had recently travelled to South Africa. Its game against Benfica over the weekend had to be abandoned at halftime for lack of players.

Quarantining also became an issue when Dutch military police had to arrest a husband and wife who left a hotel where they were being held after testing positive and boarded a plane bound for Spain.

“Quarantine is not obligatory, but we assume people will act responsibly,” spokeswoman Petra Faber said.

And after Scotland reported its first six cases, First Minister Nicola Sturgeon warned that “there might already be some community transmission of this variant.”

Taking no chances, Japan, which has yet to detect any omicron cases, reimposed border controls that it eased earlier this month for short-term business visitors, foreign students, and workers.

“We are taking the step as an emergency precaution to prevent a worst-case scenario in Japan,” Prime Minister Fumio Kishida said. The new measures begin Tuesday.

Israel likewise decided to bar entry to foreigners, and Morocco said it would suspend all incoming flights for two weeks starting Monday.

Despite the global worry, scientists cautioned that it is still unclear whether omicron is more alarming than other versions of the virus that has killed more than 5 million people.

And in some parts of the world, authorities were moving in the opposite direction.

In Malaysia, officials went ahead with the partial reopening of a bridge connecting it to the city-state of Singapore. And New Zealand announced it will press ahead with plans to reopen internally after months of shutdown, though it is also restricting travel from nine southern African nations.

Prime Minister Jacinda Ardern said that she didn’t anticipate any further restrictions and that bars, restaurants and gyms in Auckland can reopen, ending a coronavirus lockdown that began in August.

“We’ve come through the past two years of COVID in better shape than nearly anywhere in the world,” Ardern said, pointing to low death rates, a growing economy and high vaccination rates.

Dr. Francis Collins, director of the National Institutes of Health in the United States, meanwhile, said no data as yet suggests the new variant causes more serious illness than previous versions.

Collins echoed several experts in saying the news should make everyone redouble their efforts to use the tools the world already has, including vaccinations, booster shots and measures such as mask-wearing.

The US banned travel from South Africa and seven other southern African countries Monday. “It’s going to give us a period of time to enhance our preparedness,” the United States’ top infectious disease expert, Dr. Anthony Fauci, said on ABC’s “This Week.”

Fauci said it will take about two more weeks to have more definitive information on the transmissibility, severity, and other characteristics of omicron, according to the White House.