Editor’s note: This page will be updated as new data about XBB.1.5 emerges.
A new flavor of the omicron variety was identified in October 2022, and over the past few weeks it has steadily gained prominence in the United States. The subvariant is known as XBB.1.5, but has also been unofficially nicknamed “Kraken”, after the mythical sea monster.
Here’s what we know so far about XBB.1.5 so far.
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How did XBB.1.5 originate and where is it spreading?
Scientists first identified XBB.1.5 in New York state in October 2022, The New York Times reported it (opens in a new tab).
The subvariant originates from a wider branch of the omicron family tree known as “XBB”, which emerged as a result of two earlier versions of omicron – BA.2.10.1 and BA.2.75 – swapping genes, according to World Health Organization (opens in a new tab) (WHOSE). These closely related omicron subvariants had the ability to exchange genes when they infected the same person at the same time.
From their two parents, the XBB virus received mutations that helped them avoid protective antibodies acquired through previous COVID-19 infections and through vaccinations. But there was a trade-off: At the same time, XBB viruses lost some of their ability to bind tightly to cells, a key step in infection, the New York Times reported. This may explain why other versions of omicron initially outcompeted XBB virus.
However, as XBB viruses spread, they picked up new mutations and XBB.1.5, aka “The Kraken”, was born. Kraken has a mutation called F486P, which appears to restore the virus’s ability to attach tightly to cells, researchers reported Jan. 5 in research posted to the preprint database bioRxiv (opens in a new tab). (This research has not yet been peer-reviewed or published in a scientific journal.)
In a News conference on January 4 (opens in a new tab)WHO Director-General Dr. Tedros Adhanom Ghebreyesus (opens in a new tab) reported that XBB.1.5 “is increasing in the United States and Europe and has now been identified in more than 25 countries.” Genomic data submitted to the open access database GISAID (opens in a new tab) shows that the US, UK, Austria, Denmark, Canada, Israel and Germany have detected the most XBB.1.5 sequences so far, and that the subvariant remains relatively rare elsewhere.
How easily does it spread?
Available evidence suggests that XBB.1.5 is the “most transmissible” omicron progeny yet discovered, Maria Van Kerkhove (opens in a new tab), WHO’s technical lead for COVID-19, said at a press conference on January 4, according to The New York Times. In the US, XBB.1.5 is beginning to gain dominance over other circulating omicron subvariants.
As of early December, Kraken accounted for an estimated 2% of all US COVID-19 cases, It was reported by the Washington Post (opens in a new tab). That number jumped to 40% in the last week of December, STAT reported (opens in a new tab).
The Centers for Disease Control and Prevention (opens in a new tab) (CDC) has not yet analyzed all the data from early January 2023, but their current estimates suggest that XBB.1.5 accounted for more than 27% of cases in the US in the first week of the year. In the northeastern United States, where XBB.1.5 was first discovered and remains most common, the subvariant accounts for more than 70% of new cases, according to The Washington Post.
That said, nationwide, other flavors of omicron — namely BQ.1 and BQ.1.1 — were still circulating at comparable levels to XBB.1.5 during the first week of January, the CDC’s estimates suggest.
Is XBB.1.5 more likely to cause serious disease?
Researchers need to see weeks of hospitalization and death data before they can determine whether XBB.1.5 is more likely to trigger severe disease compared to earlier versions of SARS-CoV-2, the virus that causes COVID-19.
As the U.S. experiences a nationwide increase in COVID-19 infections, “we’re seeing hospitalizations increase across the country,” Dr. Barbara Mahon (opens in a new tab)said the director of the CDC’s division of coronaviruses and other respiratory viruses NBC News (opens in a new tab). “They don’t seem to nick more in the areas that have more XBB.1.5,” suggesting that the subvariant is not necessarily more likely to cause severe disease than its predecessors.
How well do boosters and treatments work against XBB.1.5?
Early data suggest that the so-called bivalent boosters — the two recently updated boosters made by Moderna and Pfizer — offer decent protection against XBB virus, despite the lineage’s ability to evade antibodies, according to a Dec. 21 report in New England Journal of Medicine (opens in a new tab).
“Laboratory studies suggest that the bivalent vaccine is still effective in protecting against severe disease, but perhaps not so much against infection,” Andy Pekosz (opens in a new tab)a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said in a statement (opens in a new tab). “XBB.1.5 is derived from the omicron variant BA.2, and while the current bivalent vaccine was developed for the BA.5 variant, it has been shown to generate antibodies that recognize BA.2,” he said.
“Things like boosters are always beneficial,” Christian Andersen (opens in a new tab), a professor in the Department of Immunology and Microbiology who tracks coronavirus variants at the Scripps Research Institute, told The Washington Post. “Even if you do get infected, you are expected to have less viral load, and you are expected to be able to transmit the virus less.”
(Notably, as of January 4, less than 16% of eligible US residents had received a bivalent booster, The CDC reported (opens in a new tab).)
Palxovid, an oral antiviral pill used to treat COVID-19, will be effective in treating infections with XBB.1.5, The New York Times reported. The pill cannot be prescribed to all COVID-19 patients, as it is not compatible with certain medications, Pakosz noted, “but overall, for the vast majority of people, Paxlovid is still a good drug to prescribe if you get COVID-19. “