Scientists and health officials around the world are closely watching the descendants of the omicron variant, which has been found in at least 40 countries, including the U.S.

This version of the coronavirus, dubbed BA.2 by scientists, is widely considered to be more stealthy than the original version of the omicron because specific genetic features make it harder to detect. Some scientists worry it could also be more contagious.

But they say they still don’t know much, including whether it’s better at evading vaccines or causing more severe disease.

Where has it spread?

Since mid-November, more than 30 countries have uploaded nearly 15,000 BA.2 gene sequences to GISAID, a global platform for sharing coronavirus data.As of Tuesday morning, 96 of those sequenced cases were from the U.S.

“So far, we haven’t seen it start an epidemic in the United States,” said Dr. Wesley Long, a pathologist at Houston Methodist Church in Texas, who has identified three cases of BA.2.

This mutant appears to be more common in Asia and Europe. In Denmark, it accounted for 45% of all COVID-19 cases in mid-January, up from 20% two weeks earlier, according to the Statens Serum Institut, part of the Danish Ministry of Health.

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What do you know about this version of the virus?

BA.2 has many mutations. About 20 of the spike proteins pinned to the outside of the virus are shared with the original omicron. But it also has additional genetic changes not seen in the original version.

Dr. Jeremy Luban, a virologist at the University of Massachusetts Medical School, said the significance of these mutations was unclear, especially in people who encountered the original omicron.

Currently, the original versions known as BA.1 and BA.2 are considered subsets of omicron. But if it’s considered a globally significant “variant of concern,” global health leaders could give it its own Greek letter name.

The rapid spread of BA.2 in some places has raised concerns that it could take off.

“We have some indication that it may be as contagious as the (primitive) omicron, or more contagious, because of its ability to compete with it in certain areas,” Long said. “But we don’t necessarily know why this is happening.”

Preliminary analysis by Danish scientists showed no difference in hospitalization rates for BA.2 compared to the original omicron. Scientists there are still studying how infectious this version is and how well current vaccines work against it. It’s unclear how well the treatment will work on it.

Doctors aren’t sure if someone who has been infected with COVID-19, which is caused by omicron, will get sick again with BA.2. But they are hopeful, especially if someone later becomes infected with BA.2, a previous omicron infection might lessen the severity of the disease.

The two versions of omicron have enough in common that infection with the original mutant “gives you cross-protection against BA.2,” said Dr. Daniel Kuritzkes, an infectious disease specialist at Brigham and Women’s Hospital.

Scientists will test to see if antibodies from the original omicron infection “can neutralize BA.2 in the lab and then extrapolate from there,” he said.

How concerned are health agencies?

The World Health Organization generally classifies omicron as a worrying variant, its most severe designation for a coronavirus mutant, but it does not designate BA.2 separately. However, given the rise of BA.2 in some countries, the agency said investigations into BA.2 “should be prioritized”.

Meanwhile, the UK Health Security Agency has designated BA.2 as a “variant under investigation”, citing increasing numbers found in the UK and internationally.Still, the original version of omicron dominates the UK

Why is it harder to detect?

The original version of omicron had a specific genetic profile that allowed health officials to use some sort of PCR test to quickly distinguish it from delta for what’s called an “S gene target failure.”

BA.2 does not have the same genetic quirk. So in testing, Long said, BA.2 looked like delta.

“It’s not that the test didn’t detect it; it just didn’t look like omicron,” he said. “Don’t think ‘stealth omicron’ means we can’t detect it. All of our PCR tests can still detect it.”

What should you do to protect yourself?

Doctors recommend taking the same precautions as before: getting vaccinated and following public health guidance on wearing masks, avoiding crowds and staying home when sick.

“These vaccines are still well protected against severe illness, hospitalization and death,” Long said. “Even if you’ve had COVID 19 before — you’ve had it naturally — the protection from the vaccine is still stronger, longer-lasting, and actually … works well for people who have had it before.”

The latest version is another reminder that the pandemic is not over.

“We all want it to end,” Long said, “but until we vaccinate the world, we’re going to be at risk of new variants.”