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XBB.1.5: What we know so far about the latest omicron subvariant

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A highly transmissible covid-19 subvariant called XBB.1.5 is now the dominant cause of covid-19 infections in the US – but there’s no evidence it causes more serious illness



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6 January 2023

Illustration of a subvariant of the coronavirus

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Joshimer Binas / Alamy Stock Photo

A new omicron subvariant called XBB.1.5 is now the dominant covid-19 strain in the US, and will likely become so in other parts of the world.

The proportion of covid-19 infections due to XBB.1.5 – nicknamed the Kraken – has been doubling almost every week in the US, says Stuart Ray at Johns Hopkins University School of Medicine in Maryland, making it the nation’s fastest-spreading variant. The US Centers for Disease Control and Prevention (CDC) estimates that more than 40 per cent of covid-19 cases in the country are due to XBB.1.5, skyrocketing from a mere 1 per cent at the start of December. In the north-eastern US, as much as 75 per cent of cases may be XBB.1.5.

“It’s the most transmissible subvariant detected yet,” said Maria Van Kerkhove at the World Health Organization in a press conference on 4 January.

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So far, 28 other countries, including the UK and Australia, have detected XBB.1.5, she said. While the UK government doesn’t publicly report covid-19 variant proportions, the Wellcome Sanger Institute, a research institution in the UK, estimates XBB.1.5 made up 4 per cent of the country’s cases in mid-December.

XBB.1.5 originated after two previous covid-19 variants swapped genetic material while infecting the same individual, says Ray. Therefore, XBB.1.5 is genetically similar to other omicron subvariants, but with a few advantageous characteristics, one of which may be an improved ability to bind to and infect human cells.

XBB.1.5 also appears to be better at evading immunity than previous variants due to changes in its spike protein, the part of the virus targeted by vaccines. A preliminary study led by Can Yue at the Chinese Academy of Sciences in Beijing found that the XBB.1.5 subvariant had an enhanced ability to escape antibodies in blood samples taken from 116 people, all of whom had previously received either three doses of the CoronoVac covid-19 vaccine or two doses of an mRNA vaccine and had recovered from a recent covid-19 infection.

However, this doesn’t mean covid-19 vaccines offer no protection against this rapidly spreading subvariant. Ample evidence indicates individuals with at least two vaccines are less likely to become seriously ill or die from covid-19 than those with fewer shots, even with newer variants, says Ray. “Antibodies to older strains get activated and augmented by exposure to current strains, even if the match isn’t 100 per cent,” says Bruce Hirsch at North Shore University Hospital in New York.

Protection is even better with the bivalent boosters available in the US and the UK, which target subvariants that more closely resemble XBB.1.5, says Ray. Only about 15 per cent of people in the US over the age of 5 have received the updated booster, leaving much of the population vulnerable.

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The good news is that many treatments for covid-19, including antiviral medications such as Paxlovid, will remain effective against XBB.1.5, says Ray. Preventative measures such as masking and improving indoor air ventilation are also able to slow the spread of XBB.1.5, he says.

“What we’re not seeing, thank goodness, is a virus that is brand new,” says Hirsch. “This is just the latest variant to become a bit more efficient and a bit more infectious.”

Reference: bioArxiv, DOI: 10.1101/2023.01.03.522427

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