EXPLAINER: What's known about 'stealth' version of omicron?

Scientists and health officials around the world are keeping their eyes on a descendant of the omicron variant that has been found in at least 40 countries, including the United States.

This version of the coronavirus, which scientists call BA.2, is widely considered stealthier than the original version of omicron because particular genetic traits make it somewhat harder to detect. Some scientists worry it could also be more contagious.

But they say there’s a lot they still don’t know about it, including whether it evades vaccines better or causes more severe disease.

WHERE HAS IT SPREAD?

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

“Thus far, we haven’t seen it start to gain ground” in the U.S., said Dr. Wesley Long, a pathologist at Houston Methodist in Texas, which has identified three cases of BA.2.

The mutant appears much more common in Asia and Europe. In Denmark, it made up 45% of all COVID-19 cases in mid-January, up from 20% two weeks earlier, according to Statens Serum Institut, which falls under the Danish Ministry of Health.

WHAT’S KNOWN ABOUT THIS VERSION OF THE VIRUS?

BA.2 has lots of mutations. About 20 of them in the spike protein that studs the outside of the virus are shared with the original omicron. But it also has additional genetic changes not seen in the initial version.

It's unclear how significant those mutations are, especially in a population that has encountered the original omicron, said Dr. Jeremy Luban, a virologist at the University of Massachusetts Medical School.

For now, the original version, known as BA.1, and BA.2 are considered subsets of...