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(NEXSTAR) – As hospitals nationwide continue struggling to operate during the omicron surge, a variant inside the variant is now raising concerns. Monday, Newsweek reported at least 92 cases of the BA. 2 omicron subvariant have been detected in over 20 U.S. states, including California and Texas.

The omicron variant is made up of three main substrains, BAs 1-3, and until Dec. 23 the World Health Organization said 99% of cases sequenced were BA. 1, Fortune reports. Last week, the subvariant overtook BA. 1 as the dominant substrain in Denmark in just two weeks. BA. 2 went from accounting for 20% of cases to accounting for 45%. Similar increases are now being seen in the U.K. and elsewhere.

Modeling by Dr. JP Weiland shows BA. 2 cases grew 90-120% faster than BA. 1, based on Danish numbers. Weiland says data doesn’t yet confirm whether increased spread is because of higher transmissibility or increased immune escape by BA. 2.

“BA. 2 is the omicron sibling to watch,” Cornelius Roemer, a computational biologist at Switzerland’s University of Basel tweeted Monday. “It started out later than BA. 1 but has been picking up pretty much everywhere I’ve looked. Don’t think I’ve found a single country where BA. 2 has gone down over time significantly.”

Some experts say BA. 2 also shows possibilities of reinfection in people who only recently had BA. 1 omicron.

Why is it called ‘stealth’?

BA. 2 is somewhat sneakier than the already sneaky BA. 1, hence its name.

In addition to its speed, BA. 2 doesn’t have a mutation that BA. 1 had – a deletion in the spike gene – which made that strain easier to identify as omicron on PCR tests. Despite some rumors, this doesn’t mean it can’t be detected, however.

“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,” Roemer tweeted. “[It’s] frustrating to see falsehood about non-detectability still around.”

Should we be worried?

Danish officials say not just yet – especially given hospitalization numbers.

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

But Dr. Eric Feigl-Ding, Federation of American Scientists epidemiologist, says swift action to quell transmission in the U.S. is needed – especially given early signs it could cause re-infection in those who’ve just recovered from BA. 1, which he calls “vanilla omicron.”

“Denmark’s CDC scientists warns specifically about this risk,” Feigl-Ding tweeted Monday. “This is why mass infection herd strategy is extremely poor choice. And why stopping transmission is critical… The coronavirus doesn’t screw around. If a new variant wins over the previous – you know it’s more aggressive in some way or another.”

Despite quick spread, many experts say it’s still too early to know if BA. 2 will cause more severe illness than omicron.