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

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


6 January 2023

2K661HF COVID variant coronavirus medical illustration 3d rendering.  BQ.1.1 highly mutated subvariant, highly infectious

Illustration of a subvariant of the coronavirus

Joshimer Binas / Alamy Stock Photo

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

The rate of Covid-19 infections due to XBB.1.5 – nicknamed the Kraken – has doubled almost every week in the US, says Stuart Ray of the 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 percent of covid-19 cases in the country are due to XBB.1.5, skyrocketing from just 1 percent at the beginning of December. In the northeastern United States, as much as 75 percent of cases may be XBB.1.5.

“It is the most transmissible subvariant discovered yet,” said Maria Van Kerkhove of the World Health Organization at a press conference on January 4.

So far, 28 other countries, including Britain and Australia, have detected XBB.1.5, she said. While the UK government does not publicly report proportions of covid-19 variants, the Wellcome Sanger Institute, a UK research institution, estimates XBB.1.5 accounted for 4 percent of the country’s cases in mid-December.

XBB.1.5 arose after two previous covid-19 variants exchanged 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 properties, one of which may be an enhanced 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 the spike protein, the part of the virus targeted by vaccines. A preliminary study led by Can Yue of the Chinese Academy of Sciences in Beijing found that the XBB.1.5 subvariant had an enhanced ability to evade 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 does not mean that covid-19 vaccines do not provide protection against this rapidly spreading subvariant. Ample evidence suggests that 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, Ray says. “Antibodies against older strains are activated and enhanced by exposure to current strains, although the match is not 100 percent,” says Bruce Hirsch of North Shore University Hospital in New York.

Protection is even better with the bivalent boosters available in the US and UK, which target subvariants more similar to XBB.1.5, says Ray. Only about 15 percent of people in the United States over the age of 5 have received the updated booster, leaving much of the population vulnerable.

The good news is that many treatments for covid-19, including antiviral drugs like Paxlovid, will remain effective against XBB.1.5, says Ray. Preventive measures such as masking and improving indoor air ventilation can also slow down the spread of XBB.1.5, he says.

“What we’re not seeing, thank God, is a virus that’s completely new,” says Hirsch. “This is just the latest variation to become a little more effective and a little more infectious.”

Reference: bioArxiv, DOI: 10.1101/2023.01.03.522427

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