The Omicron subvariant known as BA.2 or “stealth Omicron” is now the dominant version of COVID-19 worldwide, according to the World Health Organization.

WHO officials say that between Feb. 16 and March 17, BA.2 made up 86% of reported cases. That’s a huge jump from the last report, which was made on March 8. At that time, BA.2 made up just 34% of new cases.

BA.2 is on the rise in the U.S. as well, making up roughly 30% of new infections this week, according to the U.S. Centers for Disease Control and Prevention. And it’s expected to spread even more in the weeks to come.

Dr. Anthony Fauci, the White House’s top medical advisor, however, says that while an “uptick” in cases is likely, he doesn’t believe this will grow into a “surge” due to vaccination rates.

The BA.2 subvariant is between 50% and 60% more transmissible than original Omicron—which was considerably more contagious than any previous form of COVID. Despite the higher transmissibility, it has not caused more severe illnesses, so far.

Two other (older) subvariants of Omicron, BA.1 and BA1.1, made up another 13% of the global cases in the latest WHO tally.

Stealth Omicron might be infecting more people than any other variant right now, but the WHO says it’s keeping its eyes on three other COVID mutations: Two versions of the so-called “Deltacron,” which were discovered recently, and one that combines BA.1 and BA.2. Those have been given the designations XD, XE and XF.

“None of the preliminary available evidence indicates that these recombinant variants are associated with higher transmissibility or more severe outcomes,” the WHO report read.

This story was originally featured on Fortune.com