For the past  months, Omicron subvariants BA.4 and BA.5 have dominated COVID-19 cases in the U.S. But now, there’s a class of new COVID subvariants on the rise and one in particular is getting plenty of attention. It’s called XBB—or Gryphon—and there’s a chance it could overtake everything else out there and could spread wolrdwide.

XBB is getting a lot of buzz because it spreads fast—and seems to be able to evade immunity that people have built up from having a previous COVID-19 infection or getting the vaccine, says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. Still, Dr. Schaffner says, “it’s early days and we have a lot to learn.”

Here’s what we know about XBB so far, and why doctors are keeping a close eye on it.

What is the XBB COVID variant?

XBB is one of the “new class” of Omicron variants that are spreading fast right now, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. That includes BQ.1.1, BQ.1, BQ.1.3, BA.2.3.20, and XBB, he says.

“XBB is a hybrid version of two strains of the BA.2 form of Omicron,” explains Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. It’s currently “spreading efficiently in Singapore,” he adds.

The variant was first detected in August 2022 in India, and has been detected in more than 17 countries since then, including Australia, Bangladesh, Denmark, India, Japan and the U.S., per Singapore’s Ministry of Health.