What You Need to Know About the XBB.1.5 'Kraken' Variant

A new COVID-19 variant—XBB.1.5—is making headway in the United States. Also known as the “Kraken” variant, XBB.1.5 carries some key mutations that make it the “most transmissible” Omicron subvariant yet, according to the World Health Organization.  

XBB.1.5 is estimated to be responsible for 28% of all COVID-19 cases in the U.S. now, up from a mere 4% a month ago. The variant is taking a particularly strong hold in the Northeast, where it makes up 70% of all new cases.

The variant, which was first detected in New York, is out-competing other circulating Omicron variants due to its ability to evade immune responses and tightly bind to cells. XBB.1.5 could soon be more prevalent than BQ.1 and BQ.1.1, the Omicron subvariants that dominated U.S. cases in the fall.

“We do know it’s more transmissible because we’re seeing rising caseloads,” Scott Roberts, MD, an infectious disease expert and associate medical director of infection prevention for the Yale School of Medicine, told Verywell.

While XBB.1.5 can spread quickly, Roberts said that so far, it doesn’t appear to cause more severe disease than the other circulating variants do.

A Mash-Up of Two Other Omicrons

XBB.1.5 is a recombinant variant that arose from two descendants of Omicron BA.2. Recombination happens when two different versions of a virus infect a cell at the same time. In the process of replicating, these viruses may swap some genetic material, creating a new, fusion variant.

“That’s a mechanism where you can get a lot of different mutations all at once into a single virus. And that’s potentially an explanation of why XBB.1.5 has so many new mutations all at once,” Jim Boonyaratanakornkit, MD, PhD, an infectious diseases specialist at Fred Hutch Cancer Center and acting assistant professor at the University of Washington School of Medicine, told Verywell.

There haven’t been many instances of recombination in COVID-19, though the process is common in other viral infections, such as influenza. Last spring, scientists noticed the circulation of a Delta-Omicron hybrid variants, dubbed “Deltacron.”

XBB.1.5 is as good at thwarting immune defenses as its parent variant, XBB. It also carries some key mutations that give the variant a leg up at penetrating cells and replicating, making it more transmissible.

Despite the variant’s rapid rate of spread, there isn’t yet any data to indicate that XBB.1.5 causes more severe disease than the other Omicron variants.

“The advantage we have in the United States, even with XBB.1.5, is that we have this kind of mix of immunity pool in our population right now. Some people got infected with X variant or Y variant, and then all sorts of different vaccination at different time points,” Roberts said. “So, hopefully there’s this immunity wall where things won’t spread as easily here as they do in other parts of the world.”

Vaccines Will Offer Some Protection, But Tools Are Wearing Thin

Early lab studies indicate that the bivalent vaccines can induce antibodies to neutralize XBB.1.5, though the boost is relatively modest. The bivalent booster shot was designed for BA.5, and the antibodies induced from vaccination will best be able to target that variant.

“The bivalent boosters do boost the immune response against XBB.1.5, but it’s much weaker compared to the other variants,” Boonyaratanakornkit said.

About 70% of people in the U.S. have completed their primary series vaccination, but only 15% of eligible Americans have received a bivalent booster dose. Fewer than 40% of adults older than 65—those most vulnerable to severe disease—are vaccinated with the updated shot.

While the vaccines are less effective at inducing antibodies that can neutralize XBB.1.5, they can still spur other immune cells to protect against the variant, according to Boonyaratanakornkit.

Monoclonal antibodies and other treatments, meanwhile, appear to have lost much of their power to neutralize newer Omicron variants. Evusheld (tixagevimab and cilgavimab), has been shown to be ineffective against XBB, a close relative of XBB.1.5, The Food and Drug Administration said it does not expect Evusheld to neutralize XBB.1.5, though it’s awaiting further data before making a recommendation.

“With Paxlovid, there’s more optimism, because the mechanism of action of the drug is not against spike. It’s against other parts of the virus,” Roberts said. “So, my suspicion is that Paxlovid will continue to be very effective at preventing severe disease in those at risk,” Roberts said.

What This Means for the Pandemic

The U.S. is seeing nearly half a million COVID-19 cases per week, according to the CDC. During the holidays, COVID-19 hospitalizations reached their highest point in 11 months. There have been 20,000 more daily hospitalizations since Thanksgiving.

The uptick in hospitalizations is likely exacerbated by increased travel and indoor gatherings. Boonyaratanakornkit said he anticipates that XBB.1.5 could soon become the dominant variant in the U.S. It’s yet to be seen whether that will drive more hospitalizations.

“Most people are vaccinated. And then some people have the bivalent booster, and some people have been boosted with natural infection. My hope is that with all that together, the severity of COVID-19 can be mitigated,” Boonyaratanakornkit said. “If we do see rises, hopefully, they’ll be limited to rises in case counts that don’t lead to hospitalization.”

In healthy people who have some immunity from vaccination or prior infection, XBB.1.5, like other recent Omicron subvariants, may cause very mild or asymptomatic illness. Wearing masks in crowded and indoor spaces can prevent unwitting disease spread.

The best ways to quell the spread of XBB.1.5 and other variants, Roberts said, is to be up-to-date with one’s vaccinations, practice masking and social distancing, and to stay home when sick.

“We know our bread-and-butter tools work—masking, rapid testing works. We have treatments,” Roberts said. “All of that is still at play, even with this new variant.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.