The extremely contagious Omicron variant is now the dominant version of new coronavirus cases in the United States, according to the Centers for Disease Control and Prevention, and has prompted the resumption of mask mandates in some cities and states in the Northeast, where the growth in new cases has been particularly steep.
Omicron, first discovered overseas around Thanksgiving and identified in the U.S. on Dec. 1, now accounts for more than 70 percent of new U.S. cases, according to federal estimates released Monday.
The estimates underscored the rapid spread of the new variant. Two weeks ago, the C.D.C. said Omicron accounted for just 1 percent of U.S. cases; a week ago, it was about 13 percent. Delta, which for months had been the dominant form of the virus, accounted for about 26 percent of new cases over the last week, the C.D.C. estimated.
Omicron, discovered thanks to its distinctive combination of more than 50 mutations, has turned out to be highly transmissible — two to three times as likely to spread as Delta — and less susceptible to vaccines than other variants. Early cases raised hopes that it may cause milder disease than other variants, but scientists say more research is needed.
In New York, new cases have shot up more than 80 percent over two weeks. In Washington, D.C., where the mayor reinstated an indoor mask mandate on Monday, more than three times as many cases are being identified each day as at the start of December.
In Boston, where cases are also surging, Mayor Michelle Wu on Monday announced proof-of-vaccination requirements for certain indoor spaces like gyms and restaurants.
In a news conference on Monday, Mayor Bill de Blasio of New York City said that the city must “move faster” in its response to the latest wave of new cases, but insisted that another lockdown “would have horrible impacts on the people of this city.”
New York’s governor, Kathy Hochul, on Monday called the surge in new cases “a vertical increase” because “it is going straight up,” but she did not announce new restrictions. Ms. Hochul, a Democrat, appeared at her daily coronavirus briefing without the state’s health commissioner, Dr. Mary T. Bassett, who tested positive for coronavirus via a rapid test earlier in the day.
The tens of thousands of new cases in recent days have included prominent politicians and lawmakers, such as Larry Hogan, the governor of Maryland, and Senators Cory Booker of New Jersey and Elizabeth Warren of Massachusetts.
The surge is coming as many hospitals have reached capacity and governors in several states have mobilized the National Guard to help with hospital staffing shortages. As Covid-19 cases crowd into hospitals, leaders of health care facilities in multiple states have taken out newspaper ads begging local residents to get vaccinated.
“We need your help,” pleaded the leaders of several health care facilities in Ohio in a full-page ad in Sunday’s edition of the Cleveland Plain-Dealer. “We now have more Covid-19 patients in our hospitals than ever before.”
As has been the case throughout the pandemic, the situation has varied around the country, though the overall picture is growing worse by the day.
Some states are still struggling with the Delta variant, which had ravaged the South over the summer, while the Omicron variant seems to be driving the surge in the Northeast. Vaccinated people without booster shots are believed to be more vulnerable to infection by Omicron.
The six New England states — Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont — have some of the highest vaccination rates in the country, but are nonetheless seeing a worrisome new spike in coronavirus cases.
Nearly 11,000 people in the New England states are now testing positive for coronavirus every day, according to a New York Times coronavirus tracker. Almost all of these states have also seen an uptick in the number of hospitalizations and deaths over the past two weeks.
Rhode Island, with one of the highest vaccination rates in the country, with 75 percent of people fully vaccinated, is also now the U.S. state with the highest recent average cases per capita, according to the database. Daily cases have increased more than 50 percent over the last two weeks.
In response, Gov. Dan McKee announced that indoor establishments will now require masks or proof of vaccination, starting on Monday. The state is also adding more vaccine test sites for people ahead of the holidays.
“Now is the time to act,” Mr. McKee said at a Dec. 15 news conference, after describing the current pressure on the state’s hospitals, health care industry and schools. “It is not a time to just sit and wait.”
Adeel Hassan contributed reporting.