Experts are eyeing the new Omicron strain XBB.1.5—dubbed Kraken on the Twitterverse—for its potential to cause the next major COVID wave, due to its “escape strain” properties and ultrahigh transmissibility.

The World Health Organization’s technical advisory group on virus evolution is working on a risk assessment on the variant, which is the most transmissible yet, Maria Van Kerkhove, technical lead for COVID-19 response at the World Health Organization, said at a January 4 news conference. Her organization has also asked the U.S. Centers for Disease Control and Prevention to report on the risks of the new variant, since XBB.1.5 has “rapidly replaced other variants” in some European countries and in the northeast U.S.

Both reports are expected in the coming days. Right now, experts are predominantly concerned with Kraken’s ability to quickly spread and overtake other strains of Omicron.

Researchers are looking into whether XBB.1.5 might have other concerning properties, like the ability to cause more severe disease. So far, there is no evidence of this, Van Kerkhove added.

What is known, however, is that XBB.1.5 is continuing Omicron’s legacy of spawning variants that spread—and evade immunity from prior infection and vaccination—with increasing ease. Here’s what we know so far about the latest Omicron spawn to make headlines.

When and where was XBB.1.5 discovered?

While it’s only recently taken off globally, XBB.1.5 has been around for a while. It was first detected in October, according to the European Centre for Disease Prevention and Control (ECDC).

How did it evolve?

XBB.1.5 is a “recombinant”—or combination—of two spinoffs of Omicron BA.2, which was known as “stealth Omicron” because it was difficult for labs to differentiate it from Delta.

What countries has it been located in?

So far, the U.S. is seeing the most notable growth of the new variant. Last week, the CDC projected that it comprised around 75% of infections in regions 1 and 2, which include Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont, New York, New Jersey, Puerto Rico, and the U.S. Virgin Islands. Nationally, it was projected to be behind around 41% of cases.

Some European countries are also seeing a steep rise in levels of Kraken, according to the WHO. The variant has so far been reported in 25 countries, which include Denmark, France, Austria, Netherlands, Germany, Italy, Spain, Sweden, Iceland, Belgium, Czechia, Portugal, and Ireland, according to the ECDC.

Why is it so concerning?

Right now, experts are focused on the variant’s growth advantage over other strains of Omicron. In the U.S., XBB.1.5 was projected to comprise just around 1% of cases in early December 2022. A month later, it’s projected to be behind 40% of cases.

Aside from being the most transmissible COVID variant yet, XBB.1.5 also binds extra well to the cells it infects, according to WHO officials. This quality allows a virus to replicate easily in a host, which could lead to more severe disease. But so far, there is no evidence of this, WHO officials said on January 5.

Is it causing hospitalizations and deaths to rise?

The jury is still out on this. The northeast U.S., where XBB.1.5 is thriving, is seeing an increase in hospitalizations, Van Kerkhove said. The WHO also reported a 20% increase in global COVID deaths on January 5 over the past month. But it’s unknown what’s behind the trends, she said. Both could be due to increased spread that generally occurs around holidays due to gatherings, and not necessarily indicative of a new, more problematic strain.