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

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

A highly transmissible Covid-19 subvariant called XBB.1.5 is the dominant cause of Covid-19 infections in the US today – but there is no evidence it causes more serious diseases


January 6, 2023

2K661HF COVID variant coronavirus medical illustration 3D rendering.  BQ.1.1 Highly mutant subvariant, highly contagious

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 US and is likely to become so in other parts of the world as well.

The proportion of Covid-19 infections from XBB.1.5 – nicknamed Kraken – has been doubling almost every week in the US, says Stuart Ray of Johns Hopkins University School of Medicine in Maryland, making it the fastest spreading variant nationwide. The US Centers for Disease Control and Prevention (CDC) estimates that more than 40 percent of the country’s Covid-19 cases are due to XBB.1.5, up from just 1 percent in early December. In the US Northeast, up to 75 percent of cases can be XBB.1.5.

“It is the most transmissible subvariant discovered so far,” Maria Van Kerkhove of the World Health Organization said in a Jan. 4 press conference.

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

XBB.1.5 came about after two previous Covid-19 variants swapped genetic material while infecting the same person, Ray says. Therefore, XBB.1.5 is genetically similar to other omicron subvariants, but exhibits some advantageous properties, one of which could be an enhanced ability to attach 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 that vaccines target. 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 evade antibodies in blood samples from 116 people who had all previously received either three doses of the CoronoVac-Covid had received -19 vaccine or two doses of mRNA vaccine and had recovered from a recent Covid-19 infection.

However, this does not mean that Covid-19 vaccines do not offer protection against this rapidly spreading subvariant. Ample evidence suggests that people with at least two vaccines, even with newer variants, are less likely to become seriously ill or die from Covid-19 than those with fewer vaccinations, Ray says. “Antibodies against older strains are activated and multiplied when exposed to current strains, even if 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, Ray says. Only about 15 percent of people in the US over the age of 5 have received the updated booster shot, leaving much of the population vulnerable.

The good news is that many treatments for Covid-19, including antivirals like Paxlovid, will remain effective against XBB.1.5, Ray says. Preventative measures like covering up and improving indoor air ventilation are also capable of slowing the spread of XBB.1.5, he says.

“What we’re not seeing, thank God, is a brand new virus,” says Hirsch. “It’s just the latest twist to be a little bit more efficient and a little bit more contagious.”

Reference: bioArxiv, DOI: 10.1101/2023.01.03.522427

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