The more contagious version, first identified in Britain, is expected to spread widely and lead to further strains on an already overburdened health care system.
Federal health officials sounded the alarm Friday about a fast spreading, far more contagious variant of the coronavirus that is projected to become the dominant source of infection in the country by March, potentially fueling another wrenching surge of cases and deaths.
In a study released on Friday, the Centers for Disease Control and Prevention said that its forecasts indicated outbreaks caused by the new variant could lead to a burgeoning pandemic this winter. It called for a doubling down on preventive measures, including more intensive vaccination efforts across the country.
The variant is not known to be more deadly or to cause more severe disease. But the dire warning — hedged by limited data about just how prevalent the variant first identified in Britain has become — landed in a week where the nation’s nascent vaccination campaign was hampered by confusion and limited supplies as demand grew among growing numbers of eligible people
Only 76 cases of the variant have been identified so far in the U.S., but the actual number is believed to be higher and is expected to spiral upward in the next few weeks, officials said. They emphasized that current mitigation strategies were effective against the new strain, exhorting Americans to be vigilant in wearing face masks, maintaining six or more feet of distance from other people, washing hands frequently, reducing interactions with people outside their households, limiting contacts and avoiding crowds.
But spikes in cases threaten to cripple already overwhelmed hospitals and nursing homes in many parts of the country. Some are at or near capacity. Others have faced troubling rates of infection among their staff, causing shortages and increasing patient loads.
“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 C.D.C.. “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.
Covid cases and deaths have broken record after record across the country, with a peak number of deaths, 4,400, announced on Tuesday. At least 3,973 new deaths and 238,390 new cases were reported on Thursday, and the nation is nearing a milestone of 400,000 deaths.
One in 860 Americans have died of Covid-19 in the last year, according to new figures released by the C.D.C. 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, called B 1.1.7 was first identified in Britain, where it rapidly became the primary source of infections, accounting for as many 60 percent of new cases diagnosed in London and surrounding areas.
It has since been detected in at least 30 countries, including the United States and Canada. In the United States, it accounts for less than 0.5 percent of cases, based on analysis of a limited number of samples.
Other variants circulating in South Africa and Brazil are also thought to be 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 C.D.C. had announced earlier that starting Jan. 26, all air 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 the new report, C.D.C. scientists modeled how quickly the variant might spread in the United States, assuming about 10 percent to 30 percent of people have pre-existing immunity to the virus, and another 1 million people will be vaccinated each week beginning this month.
If the variant is about 50 percent more contagious, as suggested by data from Britain, it will become the predominant source of all infections in the United States by March, the model showed. A slow rollout of vaccinations will hasten that fate.
The variant differs by about 20 mutations from previous versions of the virus, including at least two mutations 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, Dr. Butler said. “C.D.C. expects these numbers to rise in the coming weeks,” he said.
State and local labs have committed to sequencing about 6,000 samples per week, a target the agency expects to hit in about three weeks.
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 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,” Dr. 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.
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