Even as coronavirus cases counts continue to dip in the United States, the prevalence of a more contagious variant of the coronavirus—called B.1.1.7, first discovered in the United Kingdom—has soared, leading some health experts to worry another surge of cases could be on the horizon.

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The spread of the B.1.1.7 variant in the US

According to data gathered by lab testing company Helix, as of last week, the B.1.1.7 variant likely accounts for more than 20% of new coronavirus cases in the United States.

To collect the data, Helix analyzed nearly 500,000 coronavirus testing samples for the S gene target failure (SGTF), an anomaly that suggests, but does not prove, the presence of the B.1.1.7 variant. It then sent samples with SGTF to the gene-sequencing company Illumina to determine what fraction actually contained B.1.1.7. Helix used that information to estimate how prevalent B.1.1.7 is in the United States.

Helix found that the relative share of the B.1.1.7 variant increased significantly over the past few weeks in nearly every state where enough data could be collected to determine a trend. For example, in Florida, where the share of B.1.1.7 cases was highest based on Helix's data, the variant was estimated to be responsible for over 30% of cases. According to the Times, that's well over the nationwide average of 20%—although the Times cautioned that Helix's testing "is not representative of population distribution."

And while B.1.1.7 is not the only variant of concern in the United States, there are more than 30 times as many cases of B.1.1.7 as of the B.1.351 variant—first discovered in South Africa—and the P.1 variant—first discovered in Brazil—according to CDC, which is monitoring confirmed cases of each variant.

In fact, CDC as early as mid-January warned that B.1.1.7 would likely become the predominant variant circulating in the United States by the end of March. So far, the agency has confirmed 2,600 cases of the variant across 46 states, Puerto Rico, and Washington, D.C.

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But the Times cautioned that there hasn't been enough genomic sequencing in the United States to get a full grasp on how prevalent the B.1.1.7 variant truly is. Just 0.5% of cumulative coronavirus cases in the United States have been sequenced since the start of the epidemic, the Times reports, and while new efforts have boosted that number to 1% in February and 3% in the past two weeks, many experts agree that about 5% of cases would need to be sequenced to provide an accurate snapshot of the variant's prevalence.

Some experts express concern about new potential surge

As the more contagious variant has spread throughout the United States, some experts have expressed concern that another surge of Covid-19 cases could follow.

"Four weeks ago, the B.1.1.7 variant made up about 1 to 4% of the virus that we were seeing in communities across the country. Today it's up to 30 to 40%," Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota, said. "What we've seen in Europe, when we hit that 50% mark, you see cases surge."

There is also concern about potential mutations of the B.1.1.7 variant as it spreads throughout the country. For instance, researchers in Oregon have spotted a "homegrown" version of the B.1.1.7 variant that contains the E484K, or "Eek," mutation also found on the B.1.351 and P.1 variants—a mutation that has been shown to make coronavirus vaccines less effective.

"We're at the point where B.1.1.7 is just being introduced" in the United States, Stacia Wyman, a computation genomics expert at the University of California-Berkeley, said. "As it evolves, and as it slowly becomes the dominant thing, it could accumulate more mutations."

Wyman added that the Oregon discovery underscores the need for people to keep taking necessary precautions to curb the virus' spread, including mask wearing, until a "substantial" share of the population has been vaccinated, the Times reports. According to CNN, fewer than 10% of all Americans are fully vaccinated.

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However, other experts say they are encouraged by the fact that coronavirus cases are continuing to drop even as variant cases surge.

"I am encouraged by the declining case counts in the most heavily affected states," Caitlin Rivers, an epidemiologist at Johns Hopkins University, said. "I've been watching Florida closely, which has the highest share of B.1.1.7. Case counts have plateaued there in recent days but are not resurging. The longer we can hold the line, the more time we have to roll out vaccines, which will protect individuals, particularly those at highest risk of severe illness, and slow transmission overall."

William Lee, VP of science at Helix, said it "may not be surprising" to see case levels rise in areas that have a high prevalence of the B.1.1.7 variant, but he added that it's possible the U.S. experience will be less severe than when the variant spiked in the United Kingdom in the winter.

"I think even if cases start going up again, the impact on hospitalizations and mortality may still be mitigated by vaccinations and higher levels of natural immunity than we had in the past," he said (Leatherby/Reinhard, New York Times, 3/6; Mandavilli, New York Times, 3/5; Maxouris, CNN, 3/8).