The World Health Organization’s technical advisory group on virus evolution is working on a risk assessment on the variant—the most transmissible yet, Maria Van Kerkhove, technical lead for COVID-19 response at the World Health Organization, said at a Wednesday 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 said.

What is known, however, is that Kraken 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 the U.S.—on Oct. 22, 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. This 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 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?

In a Wednesday tweet, Dr. Ashish Jha, the White House’s COVID czar, called XBB.1.5’s rapid U.S. rise “stunning.”

“We’ve not seen such rapid growth of a variant since Omicron BA.1 a year ago,” Dr. Eric Topol, a professor of molecular medicine at Scripps Research and founder and director of the Scripps Research Translational Institute, tweeted last week.

Kraken is estimated to have a growth advantage of around 140% over other circulating variants, the ECDC said in a Jan. 5 report, adding that the estimates come with “significant uncertainty.” The organization reports an estimated doubling time of nine days, and said the variant is likely to become dominant in the U.S. when the CDC’s new COVID forecast is released Friday.

Even if XBB.1.5 becomes dominant in the U.S., it may not in Europe, the report points out, as “major differences in variant circulation have been observed between North American and Europe several times during the pandemic.” Overall, Kraken is present in Europe at “low levels,” and is therefore unlikely to contribute to an uptick in cases there in January, the report added.

The new variant’s extreme transmissibility is likely due to its high level of immune escape, according to the ECDC. It has all the traits and mutations of its ancestor, the XBB strain, but with an additional change on the spike protein, where the virus binds to and infects human cells. This change could make XBB.1.5 more transmissible than XBB, more immune evasive, or both. The new strain is likely more immune-evasive than other strains of COVID, Jha said via tweet.

The new concerning variant also binds extra well to the cells it infects, WHO officials said Wednesday. 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.

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. And on Thursday the WHO reported a 20% increase in global COVID deaths over the past month. But it’s unknown what factors are behind the trends, according to Van Kerkhove. Both upticks could be due to increased COVID spread that generally occurs around holidays, due to indoor gatherings, and are not necessarily indicative of a new, more problematic strain.

Will the new Omicron COVID booster protect me?

It’s too soon to tell. But lab data suggests that XBB.1.5 isn’t more immune-evasive than XBB, and the booster offered some protection against XBB, Topol wrote in a recent blog entry.

In a letter published by The New England Journal of Medicine late last month, researchers from Emory and Stanford universities, in addition to the National Institute of Allergy and Infectious Disease, wrote that old monovalent vaccines and new bivalent booster vaccines, which offer protection against Omicron, were effective against XBB. But XBB limited the protection offered by the vaccine more than any other variant they tested, including BA.1, the original Omicron; BA.5, which swept the U.S. and other countries this past summer; BA.2.75; and this winter’s BQ.1.1 variant. XBB.1.5 was not tested in their study.

Protection against XBB.1.5 is probably “not that great” among those who were last infected with COVID before July and who haven’t received the new booster, Jha wrote in a recent tweet, adding that data on the vaccine effectiveness against the new variant was forthcoming.