The mutated COVID strain, which was first identified in late January, now makes up between 51-59% of all cases in the U.S. (The Delta variant, incidentally, which plagued the country last summer, made up 0% of the cases in the most recent report.)

The U.S. has been a bit behind the rest of the world, when it comes to BA.2. Last Thursday, the World Health Organization (WHO) said the subvariant had become the dominant version of COVID-19 worldwide, making up 86% of reported cases.

To underline how fast the virus is now spreading, on March 8, it made up just 34% of global cases. And at the beginning of February, BA.2 made up just 1% of sequenced COVID cases in the U.S.

The rapid spread of the subvariant does not guarantee a wave will hit the nation, though. Dr. Anthony Fauci, the White House’s top medical advisor, says that while an “uptick” in cases is likely, he doesn’t believe this will grow into a “surge” due to vaccination rates.

Also, while BA.2 is more virulent and spreads faster, it does not seem to make people any sicker than an earlier version of Omicron.

The Northeast saw the widest spread, with BA.2 cited in 70% of all cases. In the South and West, however, it still made up just over one-third of all infections last week. (Those numbers are expected to increase in the coming weeks.)

News of BA.2’s dominance comes just as the Food and Drug Administration authorized additional booster shots for Americans aged 50 or older.

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 was discovered last month, and one that combines BA.1 and BA.2.  Those have been given the designations XD, XE and XF.