Scientists and health officials around the world are tracking the BA.2 subvariant of omicron, which has been referred to as "stealth omicron" because it cannot easily be identified via PCR tests.

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What is the omicron BA.2 subvariant?

The BA. 2 omicron subvariant is a descendant of the original BA.1 omicron variant that has caused massive global Covid-19 surges. On Monday, the World Health Organization (WHO) urged researchers to prioritize the investigation of BA.2's characteristics to determine whether it poses new challenges for areas already overwhelmed by the pandemic.

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"The BA. 2 descendant lineage, which differs from BA. 1 in some of the mutations, including in the spike protein, is increasing in many countries," WHO said. "Investigations into the characteristics of BA. 2, including immune escape properties and virulence, should be prioritized independently (and comparatively) to BA. 1."

Currently, there is no evidence that BA. 2 is more transmissible or evades immunity better than BA. 1, the Washington Post reports.

In fact, experts still know very little about the transmissibility of BA.2 compared with BA.1, said Jeremy Luban, a professor of molecular medicine, biochemistry, and molecular pharmacology at UMass Medical School. And according to Luban, it is too early to determine whether vaccines and existing medications will provide adequate protection against BA.2.

Like the original omicron variant, BA.2 has many mutations, including roughly 20 found in the area targeted by most vaccines. BA.2 also has unique mutations that are not found in BA.1, which could limit the effectiveness of monoclonal antibodies, Luban said.

Further, scientists have found that BA.2 is harder to detect with PCR tests than BA.1. Although researchers were able to quickly differentiate BA.1 from the delta variant using a PCR test, the BA.2 subvariant does not possess the same "S gene target failure" seen in BA.1. As a result, BA.2 looks like the delta variant on the test, according to Wesley Long, a pathologist at Houston Methodist Hospital.

"It's not that the test doesn't detect it; it's just that it doesn't look like omicron," Long said. "Don't get the impression that 'stealth omicron' means we can't detect it. All of our PCR tests can still detect it."

Where is BA.2 circulating?

So far, BA.2 has been identified in 40 countries, including the United States. Although there are few reported cases of BA.2 in the United States, the subvariant is widely circulating in Asia and Europe. 

Throughout Europe, BA.2 seems to be the most widespread in Denmark—but experts said that could be because of the country's robust program of sequencing the virus's genome, the Post reports. On Jan. 20, health officials said that the BA.2 cases made up more than 50% of the country's omicron cases.

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In the United States, at least three cases have been found at Houston Methodist Hospital in Texas, which is currently studying the genetic makeup of virus samples from its patients, the Post reports.

"The good news is we have only three," said James Musser, director of the Center for Molecular and Translational Human Infectious Diseases Research at Houston Methodist. "We certainly do not see the 5% and more that is being reported in the U.K. now and certainly not the 40% that is being reported in Denmark."

In addition, a spokesperson for the Washington Department of Health on Monday told Fox News, "Two cases of BA.2 … were detected earlier this month in Washington."

BA.2 remains 'an open question'

Although BA.2 is now on at least four continents, experts say this new subvariant shouldn't be a cause for panic, as it is expected to be relatively mild, USA Today reports.

"I don't think it's going to cause the degree of chaos and disruption, morbidity and mortality that BA.1 did," said Jacob Lemieux, an infectious disease specialist at Massachusetts General Hospital. "I'm cautiously optimistic that we're going to continue to move to a better place and, hopefully, one where each new variant on the horizon isn't news."

Similarly, Robert Garry, a virologist at Tulane University School of Medicine, said, "Variants have come, variants have gone." He added, "I don't think there's any reason to think this one is a whole lot worse than the current version of omicron."

Still, Musser argued that BA.2 deserves close attention until scientists can learn more about it.

"We know that omicron … can clearly evade preexisting immunity" from both vaccines and exposure to other variants of the virus, he said. "What we don't know yet is whether son-of-omicron does that better or worse than omicron. So that's an open question." (Best, Fox News, 1/25; Bernstein, Washington Post, 1/24; Gleeson, Becker's Hospital Review, 1/25; AP/Modern Healthcare, 1/25; Browne, Newsweek, 1/24; Weintraub, USA Today, 1/26; Tapp, Deadline, 1/24)