Case numbers of the so-called “stealth Omicron” variant remain low, but health experts are monitoring the emergence of a new strain in the country. So far, there are fewer than 100 confirmed cases of what scientists have named the BA.2 sub-variant in the U.S., according to reporting from Washington’s KOMO News. The strain has been positively identified in Washington and Texas, among others. 

But in Europe and parts of Asia, the new variant has accounted for a growing number of cases in recent weeks. In Denmark—where daily COVID case totals are around 40,000—”stealth Omicron” has now been identified in around 65% of new infections. In the U.K., there were 426 confirmed cases of the new variant as of Jan. 21, prompting health authorities to classify BA.2 as “variant under investigation” last week.

If Europe is any indication, “stealth Omicron” could begin accounting for a larger percentage of cases in the U.S. So let’s take a deeper look at everything we know so far about “stealth Omicron,” the new  coronavirus variant to arrive in the U.S.

How is it different from Omicron?

“Stealth Omicron,” or the the BA.2 sub-strain of coronavirus differs from the more dominant BA.1 Omicron strain (what we think of as “original” Omicron) because it lacks a mutation that made the initial strain possible to detect as Omicron in PCR tests, according to the U.K. Health Security Agency. 

BA.1 contains a genetic deletion in the “S” spike gene, and allows for doctors to quickly identify and classify positive PCR tests as Omicron. BA.2 does not contain that genetic deletion, so it is harder to classify positive cases of the new variant as Omicron in PCR tests, according to the U.K. Health Security Agency. 

But some experts say this does not mean the new sub-strain evades general detection in COVID tests. It just means it is harder to classify as Omicron when it appears in positive PCR tests.

“BA.2 is detectable by PCR…Depending on the PCR test used it may not look like BA.1 (the other Omicron). But it will still give a positive result,” Cornelius Roemer, a computational biologist at Switzerland’s University of Basel, wrote on Twitter. “[It’s] frustrating to see falsehood about non-detectability still around.”

It’s still unclear how much BA.2’s mutations impact it’s severity or transmissibility, but researchers have noted that the new variant may have upwards of 28 different mutations from BA.1, the original Omicron. As a result, some are calling for the new variant to be classified as its own variant, with its own Greek letter. 

How worried should we be?

Early reports out of Denmark, where a large number of cases of stealth Omicron have been detected, suggest that the new variant does not cause an increase in hospitalization rates when compared to the original Omicron.

“Initial analysis shows no differences in hospitalizations for BA.2 compared to BA.1,” Denmark’s Statens Serum Institut, a government-run infectious disease research center, said in a statement last week. “It is expected that vaccines also have an effect against severe illness upon BA.2 infection.”

But some experts still say the new variant presents some cause for concern. Research does not clearly show that the new variant is more transmissible than Omicron—which itself is more transmissible than previous coronavirus variants—but the rising case numbers in Europe and Asia have experts on alert.

“[Consistent] growth across multiple countries is evidence BA.2 may be some degree more transmissible than BA.1,” Tom Peacock, a virologist from the Imperial College of London, tweeted on Wednesday.

Peacock did not immediately return Fortune’s request for further comment. 

What’s next?

On Monday, the World Health Organization recommended officials take additional measures to determine whether BA.2 poses new challenges compared to the original Omicron. 

“Investigations into the characteristics of BA. 2, including immune escape properties and virulence, should be prioritized independently (and comparatively) to BA. 1,” the WHO wrote. 

Only time will tell how BA.2 stacks up against the original Omicron in the U.S. 

Case numbers are still far below the BA.1 Omicron strain, and the Centers for Disease Control and Prevention says it will continue to monitor the variant as it circulates in the U.S. and abroad.

“Although the BA.2 lineage has recently increased in proportion in some countries, it remains a very low proportion of circulating viruses in the United States and globally. Currently, there are insufficient data to determine whether the BA.2 lineage is more transmissible or has a fitness advantage over the BA.1 lineage,” said Kristen Nordlund, a CDC spokesperson, in a statement to The Washington Post.