XBB.1.5: What we know so far about the latest omicron sub-variant

XBB.1.5: What we know so far about the latest omicron sub-variant

XBB.1.5: What we know so far about the latest omicron sub-variant

A highly infectious subvariant of covid-19 called XBB.1.5 is now the dominant cause of covid-19 infections in the U.S. – but there is no evidence that it causes more serious illness


January 6, 2023

2K661HF COVID variant of coronavirus medical illustration 3d rendering.  BQ.1.1 highly mutated minor variant, highly contagious

Coronavirus subvariant illustration

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 caused by XBB.1.5 – nicknamed the Kraken – is doubling in the US almost every week, says Stuart Ray of Johns Hopkins University School of Medicine in Maryland, making it the fastest-spreading variant in the country. The U.S. Centers for Disease Control and Prevention (CDC) estimates that more than 40 percent of COVID-19 cases in the country are caused by XBB.1.5, rising sharply from just 1 percent in early December. In the Northeastern United States, as many as 75 percent of cases may involve XBB.1.5.

“This is the most contagious minor variant that has been detected so far,” said Maria Van Kerkhove of the World Health Organization at a press conference on January 4.

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

XBB.1.5 was created after two previous covid-19 variants exchanged genetic material when infecting the same person, says Ray. Therefore, XBB.1.5 is genetically similar to other omicron subvariants, but has several beneficial features, one of which may be an improved ability to bind to and infect human cells.

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

However, this does not mean that the covid-19 vaccines do not provide protection against this rapidly spreading sub-variant. Abundant evidence shows that people who have received at least two vaccines are less likely to become seriously ill or die from Covid-19 than those who have received fewer vaccines, even with the newer variants, says Ray. “Antibodies against older strains are activated and enhanced by exposure 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 bivalent boosters available in the US and UK that target sub-variants that more closely resemble XBB.1.5, says Ray. Only about 15 percent of people in the United States over the age of 5 have received an updated booster, leaving a significant proportion of the population at risk.

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

“Thank God we’re not seeing a brand new virus,” says Hirsch. “It’s just the latest variant that just got a little more powerful and a little more infectious.”

Reference: bioArxiv, DOI: 10.1101/2023.01.03.522427

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