Omicron strain XBB.1.5 is spreading so rapidly that the World Health Organization is scrambling to study its risks.

The new subvariant is the most transmissible yet, Maria Van Kerkhove, technical lead for COVID-19 response at the World Health Organization, said at a December 28 news conference. So far, however, there is no evidence that the new strain will cause illness worse than other Omicron variants.

Currently, the WHO’s main concern with the variant is its ability to quickly spread compared to other strains of Omicron, officials said. XBB.1.5 has “rapidly replaced other variants” in some European countries and in the Northeast U.S., where it comprises the vast majority of infections, according to Van Kerkhove.

As of December 30, XBB.1.5 was responsible for an estimated 75% of cases in the U.S. Centers for Disease Control and Prevention’s 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, according to the federal health agency.

And it was projected to be responsible for more than 40% of sequenced COVID cases nationally, putting it on track for potential dominance when the agency updates its projections on December 30.

“In recent weeks there have been increasing reports of hospitalizations and health system pressure, particularly in temperate regions of the Northern Hemisphere, where respiratory diseases, including flu, are also circulating,” WHO Director-General Tedros Adhanom Ghebreyesus said on January 4.

The Northeast U.S.—where XBB.1.5 is responsible for most sequenced COVID cases—is seeing increasing hospitalizations, Van Kerkhove said.

But it’s too soon to conclude whether the new variant is responsible for the uptick, she said. Other respiratory viruses, like RSV and flu, are also circulating in the area, and a rise in COVID cases would be expected after holiday celebrations, regardless of a new variant appearing, she added.

Given the prevalence of XBB.1.5 in the U.S., the WHO has asked the CDC to report on the risks of the new variant. The WHO’s own technical advisory group on virus evolution is also working on a risk assessment, which is expected to be released in the coming days.

XBB.1.5, first identified in October 2022, is a combination of two spinoffs of BA.2, “stealth Omicron.” In addition to the U.S., levels of the new variant are rapidly rising in Europe, WHO officials said. So far, it’s been identified in more than 25 countries.