The new variant’s ability to tightly bind to the human cells it infects is likely contributing to its rapid rise in the U.S.—and it could lend itself to the evolution of an even more dangerous variant, researchers in China wrote.

Kraken’s ability to efficiently hijack cells may allow it to spawn mutations that escape immunity more easily, the researchers said. They cited the precedent of “stealth Omicron” BA.2 offspring. When BA.2 was, in essence, forced to compete against other COVID variants when it circulated widely last spring, it spawned BA.2.75, which was more immune evasive. And BA.2.75’s spawn BA.2.75.2 was more immune-evasive yet.

Given all this, XBB.1.5 must be “closely monitored, and the development of effective neutralizing antibodies and vaccines against XBB.1.5 is urgently needed,” wrote the study’s authors, including Dr. Yunlong Cao of Peking University in Beijing.

XBB.1.5 rendered Evusheld and bebtelovimab, antibody treatments for high-risk patients, useless, according to the study. Another similar treatment, sotrovimab, “remains weakly reactive,” and yet another, SA55, “is still highly effective” against the new variant, the authors wrote.

The study’s findings were based on testing a lab-created version of XBB.1.5 against the plasma of 116 individuals who had received three doses of Chinese-manufactured COVID vaccine CoronaVac—and who had experienced a breakthrough infection from variants BA.1, BA.5, or BF.7 afterward. The study also included 10 people who had received three or four shots of a COVID vaccine, including two or more doses of Moderna’s or Pfizer’s, and who had experienced a breakthrough BA.5 infection.

The good news: Compared with its parent, XBB.1.5 is slightly less immune evasive, the researchers found. This means the new variant shouldn’t evade prior vaccines and infections any better than a variant already in circulation.

WHO commissions risk report

XBB.1.5 was behind 18% of COVID cases nationally last week, the Centers for Disease Control and Prevention (CDC) said Friday. It’s projected to cause 28% of cases this coming week, making it the second most common variant in the U.S.—and putting it on track to become the most dominant in the States, according to a Jan. 5 memo from the European Centre for Disease Prevention and Control (ECDC).

The variant, known colloquially as Kraken, is estimated to double the number of people it sickens every nine days, according to the ECDC’s report. Experts are eyeing it because of its ultrahigh transmissibility, with the World Health Organization (WHO) announcing this week that it had commissioned a risk assessment that outlines the new threats the variant poses, if any. The WHO requested a similar report from the CDC, since the variant is dominant in the U.S. Both reports are expected in the coming days.

Kraken is the “most transmissible subvariant detected yet,” Maria Van Kerkhove, technical lead for COVID-19 response at the WHO, said at a Wednesday news conference. 

It’s unknown if the variant is contributing to a rise in hospitalizations in the northeast U.S., where XBB.1.5 is thriving, Van Kerkhove said. 

It’s also unknown how effective new Omicron boosters are against the strain.