It's an extra-contagious version of the omicron variant, but it doesn't seem to cause more severe disease.
Since it was first identified in November, BA.2 has been spreading around the globe, driving new surges in parts of Asia and Europe. It's now the dominant coronavirus version in the US and more than five dozen other countries.
It was given the "stealth" nickname because it looks like the earlier delta variant on certain PCR tests, says Kristen Coleman at the University of Maryland School of Public Health. The original omicron, by contrast, is easy to differentiate from delta because of a genetic quirk.
In rare cases, early research indicates BA.2 can infect people even if they've already had an omicron infection. COVID-19 vaccines appear just as effective against both kinds of omicron, offering strong protection against severe illness and death.
Health officials also are tracking other variants including XE _ a combination of BA.2 and BA.1, the original omicron _ that was first identified in January in the United Kingdom. The World Health Organization is keeping tabs on XE but has not yet deemed it a variant of concern or interest.