LONDON — Britain’s main nurses’ union warned Monday that exhaustion and surging coronavirus cases among medical staff are pushing them to breaking point, adding to pressure on the government for new restrictions to bring down record-high infection numbers driven by the omicron variant.

The warning throws into stark relief the unpalatable choice Prime Minister Boris Johnson faces: wreck holiday plans for millions for a second year running, or face a potential tidal wave of cases and disruption.

Many governments in Europe and the U.S. are confronting similar dilemmas over how hard to come down on omicron, which appears more transmissible than the previous delta variant that itself led to surges in many parts of the world. Early evidence suggests omicron may also produce less serious illness — though scientists caution it is too soon to say — and that it could better evade vaccine protection.

Even if it is milder, the new variant could still overwhelm health systems because of the sheer number of infections. Confirmed coronavirus cases in the U.K. have surged by 50% in a week as omicron overtook delta as the dominant variant.

Patricia Marquis, England director for the Royal College of Nursing union, said the situation over the next few weeks looks “very bleak,” as growing absences from sickness and self-isolation hit hospitals struggling to clear a backlog of postponed procedures and treat normal winter sicknesses alongside coronavirus cases.

“In many places they’re already under immense stress and pressure, and so they are starting to go off sick themselves with COVID, but also mental and physical exhaustion,” she told the BBC. “So, staff are looking forward now thinking, ‘Oh my goodness, what is coming?'”

The British Medical Association has warned that almost 50,000 doctors, nurses and other National Health Service staff in England could be off sick with COVID-19 by Christmas Day unless additional restrictions are introduced.

But many political leaders are reluctant to impose the stiff measures they resorted to earlier in the pandemic — often because they promised their people that vaccines would offer a way out of such restrictions and it may be politically untenable to impose them again.

In the U.S., the prospect of a winter chilled by a wave of coronavirus infections is a severe reversal from the optimism projected by President Joe Biden some 10 months ago, when he suggested that the country would essentially be back to normal by this Christmas. France is desperately trying to avoid a new lockdown that would hurt the economy and cloud President Emmanuel Macron’s expected re-election campaign.

Britain’s Johnson, whose authority has been hammered by weeks of political scandals, is caught between calls from scientific advisers for new limits on social interaction now, and vociferous opposition within his Conservative Party to any such restrictions.

Earlier this month, Johnson’s government reinstated rules requiring face masks in shops and ordered people to show proof of vaccination or a negative coronavirus test before entering nightclubs and other crowded venues.

The government is hoping vaccine boosters will offer more protection against omicron, as the data suggests, and has set a goal of offering everyone 18 and up an extra shot by the end of December. More than 900,000 booster shots were delivered on Sunday, as soccer stadiums, shopping centers and cathedrals were turned into temporary inoculation clinics.

But many scientists say tougher action is needed.

U.K. Deputy Prime Minister Dominic Raab said Monday he could not “make hard and fast guarantees” that new restrictions would not be announced this week.

Government ministers are discussing several options, ranging from non-binding guidance for people to limit festive gatherings to mandatory social distancing and curfews for bars and restaurants.

The speed of omicron’s spread in the U.K., where cases of the variant are doubling about every two days, is decimating the economy in the busy pre-Christmas period.

Usually teeming theaters and restaurants are being hit by cancellations. Some eateries and pubs have closed until after the holidays because so many staff are off sick or self-isolating. The Natural History Museum, one of London’s leading attractions, said Monday it was closing for a week because of “front-of-house staff shortages.”

The hospitality industry is urging the government to offer financial support, as it did earlier in the pandemic when many businesses were forced to shut or severely curtail their activity. Those programs, including grants, loans and a scheme that paid the salaries of millions of furloughed workers, were wound down after Britain lifted restrictions in the summer.

Other countries are warily watching the U.K., which on Sunday reported 82,886 more lab-confirmed COVID-19 cases in a day, close to a record high.

The Dutch government began a tough nationwide lockdown on Sunday to rein in sharply rising infections. But other European countries have opted for something less.

France and Germany have barred most British travelers from entering, and the government in Paris has also banned public concerts and fireworks displays at New Year’s celebrations. Ireland imposed an 8 p.m. curfew on pubs and bars and limited attendance at indoor and outdoor events.

In Spain, the national average of new cases is double what it was a year ago. But authorities in the country with one of Europe’s highest vaccination rates are betting primarily on mandatory mask-wearing indoors and the rollout of booster shots, with no further restrictions in the pipeline.

Miguel Hernán, an epidemiologist at Harvard University who advised the Spanish government earlier in the pandemic, said the reluctance to impose new curbs was understandable, but warned it was better to act early rather than late.

“The earlier they are applied, the shorter the time they are needed,” he said.

German Health Minister Karl Lauterbach said Sunday that there won’t be a lockdown before Christmas but cautioned: “We will have a fifth wave.”

Hendrik Wuest, governor of North Rhine-Westphalia, Germany’s most populous state, said more restrictions could be on the horizon shortly after Christmas.

“I don’t think big New Year parties can happen this year — unfortunately, again,” he added. “Omicron won’t forgive us any carelessness if we aren’t cautious.”