Federal health officials warned Friday that a far more contagious variant of the coronavirus first identified in Britain could become the dominant source of infection in the United States by March, and would likely lead to a wrenching surge in cases and deaths that would further burden overwhelmed hospitals.

This dire forecast from the Centers for Disease Control and Prevention made plain what has been suspected for weeks now: The nation is in an urgent race to vaccinate as many Americans as possible before the variant spreads across the country.

Public health officials emphasized that protective measures already in place should work against the new variant, and urged Americans to redouble their vigilance in wearing face masks, in maintaining physical distance outside their households, washing hands frequently and limiting social interactions and indoor gatherings.

The variant is not known to be more deadly or to cause more severe disease. But the worrisome warning — hedged by limited data about just how prevalent the variant has become — landed at the end of a week when the nation’s nascent vaccination campaign appeared to be scattershot and still disappointingly elusive for most Americans. It was hampered by confusion over eligibility for people beyond front-line health workers, miscommunication over increasingly limited supplies as demand grew and by bungled rollouts from state to state.

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The images of elderly Americans standing or sitting for hours in long lines, anxious for their shots, while some were turned away became emblematic of a patchwork approach that belied the promises of protection for the most vulnerable.

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The CDC’s projections could also prove extremely troubling for hospitals and nursing homes, many of which are already operating at or near capacity. Medical centers and nursing homes have faced increasing rates of infection among their staffs, causing shortages and leading to increased patient loads that have at times jeopardized patient care.

“I want to stress that we are deeply concerned that this strain is more transmissible and can accelerate outbreaks in the U.S. in the coming weeks,” said Dr. Jay Butler, deputy director for infectious diseases at the CDC. “We’re sounding the alarm and urging people to realize the pandemic is not over and in no way is it time to throw in the towel.”

“We know what works and we know what to do,” he said.

The agency’s study lends urgency to the plan announced by President-elect Joe Biden, who is proposing to spend more than than $400 billion to combat the pandemic and accelerate vaccine distribution. It is part of his larger $1.9 trillion economic package aimed at offering financial aid and relief to local governments facing shortfalls and to individuals, and businesses that sustained losses during the nearly yearlong crisis.

“The more people we vaccinate and the faster we do it, the sooner we can put this pandemic behind us and the sooner we can build our economy back better and get back to our lives and our loved ones,” Biden said Friday as he announced a five-point vaccination plan.

Privately, one CDC official said the prospect of the new variant’s prowess was “chilling,” and underscored the urgent need for people to follow precautions. A bulletin released by the agency — which used highway emergency sign imagery to warn of rising cases, strained hospitals and new more contagious variants — conveyed the sense of urgency. “More spread, more cases, more deaths,” it warned.

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COVID cases and deaths have broken record after record across the country, with a peak number of deaths, 4,400, announced Tuesday. At least 3,973 new deaths and 238,390 new cases were reported Thursday, and the nation is nearing a milestone of 400,000 deaths.

One in 860 Americans have died of COVID in the last year, according to new figures released by the CDC. But the burden of deaths has not fallen equally across racial, ethnic lines and geographic regions, and there is concern that vaccines will not reach the hardest hit communities, where access to health services is limited and distrust is rampant.

The new variant could further exacerbate health disparities among communities of color, some experts warned.

“I see the new strain as a threat multiplied. Take everything we know about the risk of this virus and just multiply it substantially,” said Dr. Ashish K. Jha, dean of Brown University’s School of Public Health. He expressed concern that the vaccines were not reaching vulnerable communities and called for locating immunization sites in communities of color and using public health messages to alleviate vaccine hesitancy. He also proposed vaccinating all people over the age of 55 after health care workers and those in long-term care facilities are inoculated.

The new variant, called B.1.1.7, was first identified in Britain, where it rapidly became the primary source of infections, accounting for more than 80% of new cases diagnosed in London and at least a quarter of cases elsewhere in the country.

It has since been detected in at least 50 countries, including the United States and Canada, according to the World Health Organization. In the United States, it accounts for less than 0.5% of cases, based on analysis of a limited number of samples.

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Other variants circulating in South Africa and Brazil are also considered more contagious, but have not yet been identified in the United States. Japanese authorities said this month that they had detected one of the variants in four passengers arriving from Brazil.

The CDC had announced earlier that starting Jan. 26, all airline passengers arriving in the United States, regardless of vaccination status, would be required to show proof of a negative result from a test for the coronavirus or of recovery from COVID.

In Britain, infections also spiked in children of all ages, fueling fears that the new variant would be just as dangerous in children as in adults, and forcing Prime Minister Boris Johnson to shut down all schools. But while the new variant is more contagious than previous iterations of the virus, children are still only about half as likely as adults to spread it to others, experts have said.

In the new report, CDC scientists devised a model to assess how quickly the variant might spread in the United States, assuming about 10% to 30% of people have preexisting immunity to the virus, and another 1 million people a day will be vaccinated beginning this month.

If the variant were about 50% more contagious, as suggested by data from Britain, it would become the predominant source of all infections in the United States by March, the model showed. A slow rollout of vaccinations would hasten that fate.

“We know that that’s an overestimate of the current level of vaccination that’s occurring,” said Michael Johansson, a researcher at the CDC. “But certainly, we hope that we get to levels that are higher than that by the time that this period ends.”

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All viruses accumulate mutations over time; most of the mutations disappear, but those that confer an advantage — greater contagiousness, for example, or faster replication — may take root and spread. A more transmissible variant, in particular, is likely to spread quickly through a population.

The new coronavirus has accumulated mutations of concern faster than many researchers had anticipated. Some variants also contain mutations that may slightly weaken the protection from vaccines.

But the immunity produced by vaccines is extremely powerful and should remain effective for years, said Paul Duprex, the Jonas Salk Chair for Vaccine Research at the University of Pittsburgh. “It’s not going to go from being a 94% efficacy to a 32% vaccine efficacy overnight,” he said.

The variant identified in Britain differs by about 20 mutations from previous versions of the virus, including at least two mutations that may contribute to its greater contagiousness. As of Jan. 13, it had been detected in 76 cases from 12 states, but the actual numbers are likely to be much higher, Butler said. “CDC expects these numbers to rise in the coming weeks,” he said.

The CDC has sequenced about 71,000 samples of the virus, a minuscule fraction of the 23 million people infected in the country to date. But the agency has ramped up its efforts by about sixfold in the past two weeks in light of B.1.1.7 and other variants, said Dr. Gregory Armstrong, who leads molecular surveillance efforts at the agency.

State and local public health labs have committed to sequencing about 6,000 samples per week, a target they expect to hit in about three weeks.

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Agency officials also warned that standard tests for the virus may miss one of the altered genes in the new variant. That should not be an issue for most laboratory-based PCR tests, they said, but some antigen tests may produce “false negatives,” missing cases of infection.

“So far, we haven’t found evidence of that, but we’re looking more closely at that,” Butler said.

It’s not yet clear what makes the new variants more contagious. They share at least one mutation, called N501Y, that is thought to be involved. One possibility, researchers said, is that the mutation may increase the amount of virus in the nose but not in the lungs — potentially explaining why it is more contagious, but not more deadly.

A higher amount of virus in the nose means anyone infected would expel more virus while talking, singing, coughing or even breathing, said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

“It makes the same situations that generate spread now — people living in the same household, these sorts of non-ventilated indoor contacts — to be more likely to spread,” he said.