Stealth Omicron, also known as the Omicron BA.2 variant, was first detected in Europe in late January and has since made its way around the world, becoming the leading strain behind new coronavirus infections in at least 18 countries.

Research is trickling in about just how concerning the new variant is.

How deadly is the new COVID strain?

A study from Denmark, where stealth Omicron rapidly became the dominant lineage of the virus, sampled 263 cases of COVID reinfection from stealth Omicron in the country, and found that reinfections were nearly four times as common for people who had recovered from the Delta variant than those who had come down with Omicron.

The good news from the Danish study is that prior infection with Omicron and vaccination seemed to be enough to provide abundant protection to the new strain.

Other studies have also found that antibodies from the original Omicron strain were able to provide strong protection against stealth Omicron. This survey cataloging COVID reinfections in the U.K. from early February did not identify any cases where a stealth Omicron reinfection followed an Omicron infection.

Eric Topol, a genomicist at Scripps Research in La Jolla, Calif., said the new research was “reassuring” to him. “Instead of thinking that [stealth Omicron] is the new bad variant, I think we can put that aside. I see it as not a worry,” Topol told Nature.

How contagious and transmissible is it?

But while initial data is still spotty on whether stealth Omicron can cause more serious disease than its parent strain, it is very likely more transmissible. A study in Japan released at the end of February found that the BA.2 strain could be as much as 30% more transmissible than the original Omicron variant, which was already the most contagious form of the disease we have encountered so far during the pandemic.

Some scientists have argued that stealth Omicron deserves its own Greek letter name in the coronavirus lexicon. But while BA.2 exhibits some divergent mutations from the original Omicron variant, scientists have been more comfortable calling it a subvariant, as it still has many similar characteristics to its parent strain.

The similarities to Omicron have made it more difficult to sequence and trace BA.2, hence its moniker as stealth Omicron. It was relatively easy to differentiate the Omicron variant from Delta infections through PCR tests and genetic sequencing, but the similarities between Omicron and stealth Omicron have made it much more difficult to trace the latter’s spread.

By the last week of February, the Centers for Disease Control and Prevention estimated that stealth Omicron was causing 8% of all new COVID infections, at a rate doubling every week.