Biden asks the Supreme Court to revive vaccine mandate for health care workers.

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Mayor Bill de Blasio announced new measures to combat the city’s latest coronavirus surge, including increased testing and distribution of free masks.Carlo Allegri/Reuters

Offices are canceling holiday parties. Broadway performances are being shuttered because of breakthrough infections among the casts. College finals are being moved online, and some colleges and universities are switching back to remote instruction after the winter break, at least for a time.

Coronavirus cases are spiking again in New York City, beyond anything seen since last winter, and in much of the United States, where the Delta variant remains by far the dominant version. The country is reporting more than 120,000 new cases a day on average, according to a New York Times database — a 40 percent increase from two weeks ago, and 70 percent more than when cases stopped falling in early November.

While most people who are unvaccinated remain at much higher risk, a combination of factors — widespread transmission, waning immunity and the circulation of highly infectious variants — has led to breakthrough infections among the vaccinated.

Several new laboratory studies indicate that vaccines, and especially boosters, may offer substantial protection against the worst outcomes from the fast-spreading Omicron coronavirus variant. Still, the new variant will infect many people who have been vaccinated or recovered from older versions of the virus, according to the research.

The United States passed two milestones this week — 50 million total known cases and 800,000 total deaths — and experts say the addition of Omicron is likely to push the trajectory up even more. So will the holiday season, public health officials say, as people travel and families gather indoors, where the virus spreads most readily.

Outbreaks are growing most severely around the Great Lakes and in the Northeast. Connecticut is averaging more than 2,600 new cases a day, up from about 330 at the start of November, and Rhode Island is adding cases at the highest rate in the country. Michigan, Indiana, Ohio and Pennsylvania have the highest rates of coronavirus hospitalizations.

In some hard-hit states, federal medical workers have been sent to help exhausted hospital staff. About 68,000 coronavirus patients are hospitalized nationwide, a 21 percent increase from two weeks ago, but well below the levels seen over the summer and at last winter’s peak.

Biden administration officials are bracing for a potential wave of Omicron infections that could overwhelm the health care system. The Centers for Disease Control and Prevention warned recently that the percentage of coronavirus cases in the United States caused by Omicron had increased sharply and might portend a surge in infections as soon as next month.

The administration is trying to encourage all Americans who are eligible — those 16 and older who received their second Pfizer-BioNTech or Moderna vaccine dose at least six months ago, or the single-dose Johnson & Johnson shot at least two months ago — to get a booster.

Across the country, cultural institutions and many ordinary Americans are reconsidering what is safe. The Metropolitan Opera announced on Wednesday that, starting next month, it would require booster shots for all eligible employees and audience members. The N.F.L. and N.B.A. are discussing changes to their protocols in response to a wave of infections among players. Some Wall Street banks are telling workers who had been going into the office to stay home over the holidays.

Similar surges, and accompanying crackdowns, are happening in many countries, especially in Europe. The French government on Thursday banned nonessential travel to and from Britain and tightened testing requirements for travelers, as a record-breaking surge of Omicron cases in Britain raised worries that France would soon face a similar tsunami of new infections.

As of Thursday, an average of 3,700 new cases per day were being reported in New York City, a level not seen since early April. New York State, which reports cases before the city does, showed caseloads skyrocketing further, with more than 18,000 new cases on Thursday, 8,300 of them in New York City — levels not seen since the holiday spike last January.

New York’s governor, Kathy Hochul, announced that the state would let New Yorkers in areas with low vaccination rates and high case numbers request home delivery of rapid tests, and Mayor Bill de Blasio of New York City announced a six-step plan to combat the surge, including more testing and enforcement of mask mandates.

Officially, the city’s monitoring system had only picked up the Omicron variant in about 1 percent of cases it has analyzed as of the week of December 4. But the C.D.C. recently reported in that in the region encompassing New York, New Jersey, Puerto Rico and the U.S. Virgin Islands, about 13 percent of cases were Omicron.

“We have seen a very substantial increase in Covid cases in the last few days. It is clear that the omicron variant is here in New York City in full force,” Mayor Bill de Blasio said on Thursday.

Sarah Gancher, a teacher at The New School who lives in Queens, said eight of her friends who were fully vaccinated had tested positive this week. The speed at which cases have surged has frightened her, she said.

“It’s literally all I’ve been thinking about,” Ms. Gancher said.

Sitting on a bench in Madison Square Park in Manhattan on Thursday morning, she opened her bag to show the five at-home testing kits she had just bought. Several holiday parties she had planned to attend this weekend had been canceled, and although she had tickets booked to fly home to California to visit her family, she said she wasn’t sure how safe that would be anymore.

“I’m really heartsick and worried,” she said.

Ashley Wong, Mitch Smith and Joseph Goldstein contributed reporting.

Correction: 

An earlier version of this item contained a photo with an erroneous caption. The photo of a woman wearing a face shield in Union Square was from November 2020, not November 2021. The picture has been replaced.