The Omicron subvariant BA.2, nicknamed the “stealth Omicron,” appears to be outpacing other substrains of Omicron in some regions of the world, raising fears that the even more transmissible version of Omicron could spark larger COVID-19 waves globally.

The World Health Organization (WHO) says that Omicron, which is also referred to as B.1.1.529, has three main substrains: BA.1, BA.2, and BA.3. As of Dec. 23, the WHO reported that over 99% of the cases it sequenced were BA.1. But now the rise of BA.2 in Denmark and elsewhere suggests that BA.2 may outcompete BA.1.

On Thursday, Denmark reported that the BA.2 substrain of Omicron accounts for almost half of the country’s cases and is quickly displacing BA.1, the original Omicron strain. Denmark reported that in the two weeks from late December to mid-January, BA.2 has gone from accounting for 20% of Denmark’s COVID-19 infections to making up 45%. Over that same period, Denmark’s COVID infections have shot to record highs. Denmark is recording over 30,000 new cases per day this week, 10 times more cases than peaks in previous waves.

Denmark’s government also said the strain is spreading quickly in countries like the U.K., Norway, and Sweden. Meanwhile, scientists in places like France and India warn that the BA.2 variant is quickly spreading and may outpace other Omicron strains.

But Danish authorities also urged the public to not read too much into BA.2’s rise at this point.