A new coronavirus subvariant, called XE, contains a combination of the omicron variant BA.1 and the BA.2 subvariant, known as "stealth" omicron. And while it has infected around 600 people in the United Kingdom, experts say "there's still a lot we don't know about XE."

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What is XE?

The XE subvariant contains a combination of the omicron variant BA.1, which first surfaced in late 2021, and the BA.2 subvariant, known as "stealth" omicron—currently the dominant variant around the world.

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XE is a "recombinant" virus—a type of variant that can occur when an individual has a coinfection of two or more variants at the same time, mixing the variants' genetic features within a patient's body. Notably, XE is the fifth recombinant virus that has stemmed from omicron, USA Today reports.

Although data on XE's severity and ability to evade immunity is not yet clear, early estimates suggest it could be more contagious than earlier strains, CNBC reports.

According to data from the UK Health Security Agency (UKHSA), XE has a growth rate that is 9.8% higher than BA.2—a figure the World Health Organization estimates to be 10%.

"This particular recombinant, XE, has shown a variable growth rate and we cannot yet confirm whether it has a true growth advantage. So far there is not enough evidence to draw conclusions about transmissibility, severity or vaccine effectiveness," said UKHSA chief medical advisor Susan Hopkins.

Where has XE been detected?

In the U.K., the earliest confirmed case of XE has a specimen date of Jan. 19, which means it could have been circulating in the population for several months, CNBC reports. In total, UKHSA has confirmed 637 cases of XE. It has since also been identified in Thailand.

The U.K. is currently facing a Covid-19 surge, largely driven by the BA.2 subvariant, and the XE variant currently accounts for less than 1% of total cases that have gone through genomic sequencing in the UK.

Are experts worried about XE?

Jeremy Luban, an infectious disease specialist at the University of Massachusetts Medical School, said he is monitoring XE but is not concerned yet.

 "XE has a powerful combination of all these variants of coronavirus. Should it spread rapidly through the U.S. at some point, we would be concerned," Luban said.

Separately, William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine, said there is no reason to panic.

"There's still a lot we don't know about XE," Schaffner said. "XE could surprise us, so we have to wait to see how severe it will be." 

Notably, Schaffner said he is more concerned about BA.2, which currently accounts for 72% of Covid-19 cases in the United States and remains the dominant variant around the world.

On Tuesday, CDC Director Rochelle Walensky noted that even though BA.2 has sparked several outbreaks around the world, it has not caused a significant increase in cases in the United States. In fact, Walensky said cases dropped 4% from the previous week, and hospitalizations and deaths both declined by over 16%.

According to Schaffner, variants will continue to pose a risk to the world's "return to normalcy" because of their unpredictability. "This is our new reality, variants popping up everywhere because fortunately, we are detecting them," he said. "The more we can find, the more we can study and contain." (Gilchrist, CNBC, 4/6; Miranda, USA Today, 4/7)