Israel banned foreign nationals from entering as more countries reported their first cases of the Omicron variant, raising the question of whether the fast-spreading strain will worsen a potentially severe winter wave and presenting officials with difficult policy choices.

Australia, the Netherlands and Austria joined a group of countries including the U.K., Germany, Belgium, Israel and Italy that have detected a strain that authorities say could pose a greater risk of people falling ill with Covid-19 a second time and could...

Israel banned foreign nationals from entering as more countries reported their first cases of the Omicron variant, raising the question of whether the fast-spreading strain will worsen a potentially severe winter wave and presenting officials with difficult policy choices.

Australia, the Netherlands and Austria joined a group of countries including the U.K., Germany, Belgium, Israel and Italy that have detected a strain that authorities say could pose a greater risk of people falling ill with Covid-19 a second time and could be more transmissible than other variants.

New York’s governor over the weekend declared a state of emergency to prepare for a possible Covid-19 surge. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview on ABC Sunday that “If and when, and it’s gonna be when, it comes here” the country should be prepared by getting more people vaccinated, and making sure people follow mask guidelines.

Dutch health authorities said Sunday they had confirmed 13 cases of the new variant among 61 positive Covid-19 tests from passengers on two flights that arrived in the Netherlands from South Africa on Friday. It was possible that further sequencing would find more cases of Omicron, they said.

In the Netherlands, passengers traveling from South Africa were transported to a hotel for quarantine on Saturday.

Photo: laurens bosch/Shutterstock

Nearly two years after the first confirmed outbreaks of Covid-19 in China, the rapid spread of a new variant in southern Africa shows how the coronavirus can still thwart the best efforts of policy makers world-wide to return to a post-pandemic normal.

Further analysis of the Omicron variant will determine whether governments will react by reimposing unpopular and economically damaging restrictions on their populations.

On Wednesday, China published a report saying that the country could experience the “real possibility of a colossal outbreak” if it relaxed its zero-tolerance policies and pursued those followed by Western countries, including the U.S. That report came before the World Health Organization declared Omicron a variant of concern.

After detecting one case of the new strain on Friday, Israel banned all foreign nationals from entering the country on Sunday and reinstated a controversial contact-tracing surveillance program. Israel’s health ministry also said Israelis would be prohibited from traveling to 50 African countries.

The country is now testing seven other people for the new variant, three of whom have received three shots of the vaccine and four of whom have been vaccinated twice or recovered from the virus more than six months ago, meaning they may have lost some protection against the virus.

Along with new travel restrictions, Israel’s cabinet voted to allow the country’s internal security service, the Shin Bet, to renew the use of contact tracing through cellular location services for cases related to the Omicron variant. Israeli civil- and human-rights groups have condemned the involvement of the Shin Bet, whose mandate is to fight terrorism, as a violation of privacy.

Despite a wide-scale booster shot campaign, compulsory indoor masking and a Covid-19 passport program already in place, Israeli officials say that a recent trend of an increase in new daily coronavirus cases means the country could be on the verge of another surge in infections. Israel’s population is around 9.3 million people.

Austrian authorities said late Saturday that they have detected a suspected case of the variant in a traveler who returned from South Africa last week, though confirmation requires further sequencing. The infected individual, who currently shows no symptoms, received a second vaccination with an mRNA vaccine around nine months ago, authorities said.

New regulations will require Israelis returning from abroad to quarantine for at least three days, among other measures.

Photo: abir sultan/Shutterstock

Australia recorded its first two cases of the variant in two travelers who arrived in Sydney on a flight from southern Africa on Saturday. Health authorities said the pair were asymptomatic and had been fully vaccinated. They were tested for Covid-19 on arrival in Australia.

If scientists confirm early indications that Omicron is more transmissible than the Delta variant and is better able to infect people a second time, it raises new questions on how societies will manage to live with Covid-19 in the long term.

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“How much more can society take?” asked Simon Quijano-Evans, chief economist at Gemcorp Capital LLP in London, in an email to clients Saturday, without answering the question.

In the U.K., which has tolerated relatively high numbers of Covid-19 cases since the summer largely without reimposing restrictions, the government on Saturday said it would once again require masks in shops and on public transportation and require anyone entering the country to isolate until a PCR test shows a negative result.

Since the summer, a number of European Union countries had gained some measure of control over the virus by retaining mask mandates and deploying measures—such as vaccine requirements for recreation and even to enter the workplace—aimed at pressuring people to get the shot.

In the U.K., the government said Saturday it would reimpose mask mandates in shops and on public transportation.

Photo: Matt Dunham/Associated Press

But even before Omicron emerged, a surge in the EU in cases, which have overtaken the U.S. on a per capita basis, had shaken policy makers’ confidence, pushing some authorities to reimpose lockdowns and curfews and make vaccines obligatory for virtually everyone.

Meanwhile, Omicron could cause Asian countries that have pursued zero-tolerance policies to reconsider recent, tentative reopening plans.

On Saturday, the Australian government tightened border restrictions, barring those who aren’t Australian citizens, permanent residents or their immediate family from entering the country if they have been in several African countries within the past 14 days. Those countries include South Africa, Namibia, Zimbabwe and Botswana.

Australia in November reopened its international border, ending one of the world’s strictest pandemic-era travel bans as authorities pivot from trying to suppress Covid-19 to living with it.

Last week, China’s Centre for Disease Control and Prevention published a report saying that the country could face more than 630,000 Covid-19 infections a day if it dropped its ultrastrict containment measures, including travel curbs, and adopted pandemic measures followed by the U.S. and other Western countries.

Researchers used data from August from the U.S., the U.K., France, Israel and Spain to assess the impact on China if it adopted the same pandemic policies as those countries.

It said that it could see more than 10,000 severe cases a day, potentially exerting “a devastating impact on the medical system of China and cause a great disaster within the nation.”

—Georgi Kantchev in Berlin and Alice Uribe in Sydney contributed to this article.