The new Covid XBB.1.5 variant and why it is spreading so fast


For weeks, scientists have watched a slew of Omicron descendants struggle for dominance of US Covid-19 transmission, with the BQs — BQ.1 and BQ.1.1 — appearing to outstrip all others to claim a slight lead .

The result was a gradual increase in cases and hospitalizations that never seemed to reach the peaks of this summer’s BA.5 wave and was certainly nothing like the tsunami of disease caused by the original Omicron strain a year ago.

But on Friday, the US Centers for Disease Control and Prevention’s Covid-19 variant dashboard revealed a new dark horse that may soon sweep the field: XBB.1.5.

The CDC estimates that XBB.1.5 has more than doubled its share of the Covid-19 pie each week over the past four, rising from about 4% to 41% of new infections in December. In the Northeast, the CDC estimates that XBB.1.5 is causing 75% of new cases.

“We haven’t seen a variant take off at that speed in a few months,” said Pavitra Roychoudhury, director of Covid-19 sequencing at the University of Washington School of Medicine’s laboratory of virology.

Virologists and epidemiologists say this Omicron sublineage has characteristics that give it the potential to drive a new wave of Covid-19 cases in the US, though it’s not yet clear how big that wave will be and whether it could send many more people. at the hospital.

Despite all the recent concern that a new Covid-19 threat could come from China’s ongoing rise, experts point out that XBB.1.5 appears to have arisen in the United States. It was first detected in New York and Connecticut in late October, according to GISAID, a global effort to catalog and track variants of the coronavirus.

Trevor Bedford, a professor of computational biology at the Fred Hutchinson Cancer Center in Seattle, said XBB.1.5 has a similar growth rate to its distant cousin BA.5.

Bedford pegged its effective reproductive number — the number of new infections expected to be caused by each infected person — at about 1.6, about 40 percent higher than its closest competitor.

“I expect circulation to pick up over the next few weeks,” Bedford wrote in an email. That increase may not be reflected in the number of cases, she pointed out, since more people are testing at home and their cases may not be counted unless they seek medical help and get a lab test to confirm their results. “So I would look at hospitalizations in vulnerable age groups [such as seniors] as the best indicator of the wave,” he wrote.

XBB.1.5 is the product of recombination: Two descendants of BA.2, the subvariant that caused a modest surge of cases in the US in April, swapped pieces of their genetic code, resulting in 14 new mutations against the protein virus spike with BA.2 and a new sublineage, XBB.

XBB led a surge in cases in Singapore this fall but never gained much traction in the United States. Here, it had to compete with a slew of co-circulating variants that had each independently evolved some of the same mutations, making them more evenly matched.

However, scientists have been keeping a close eye on XBB and its spin-offs.

Dr. David Ho, a professor of microbiology and immunology at Columbia University, recently tested viruses engineered to have the spikes of XBB and XBB.1 as well as BQ.1 and BQ 1.1 in his lab against blood antibodies from people who had been infected, who had been vaccinated with the original and new bivalent vaccines and who had been both infected and vaccinated. His team also tested 23 monoclonal antibody treatments against these new stresses.

He found XBB.1 to be the most slippery of all. It was 63 times less likely to be neutralized by antibodies in the blood of infected and vaccinated people than BA.2 and 49 times less likely to be neutralized than BA.4 and BA.5.

In terms of immune evasion, Ho says, these variants have strayed as far from the antibodies we created to use against them as the original Omicron variant came from the Covid-19 viruses that preceded it about a year ago.

He calls these levels of immune evasion “alarming” and said they could further undermine the effectiveness of Covid-19 vaccines. His findings were recently published in the journal Cell.

I said on Monday that XBB.1.5 had the same antibody evasion story as XBB.1, meaning it has the potential to escape the protections of vaccinations and past infections. It is also resistant to all current antibody treatments, including Evusheld.

In addition to being highly immune evasive, XBB.1.5 has one more trick up its sleeve that appears to be helping fuel its growth. It has a key mutation at site 486, which allows it to bind more tightly to ACE2, the gates the virus uses to enter our cells.

“The mutation is clearly allowing XBB.1.5 to spread better,” Jesse Bloom, a computational virologist at Fred Hutchinson Cancer Center, wrote in an email.

This mutation was first reported by Bloom, who studies the evolution of viruses and viral proteins, as a mutation that may be important for viral fitness. It was confirmed by Yunlong Cao of Peking University.

“It has a better ability to enter cells,” Roychoudhury said, which means it’s more contagious.

However, experts say it’s difficult to know how much of XBB.1.5’s growth can be attributed to the properties of the virus and how much simply to good timing.

Coming off the holidays, during which people were more likely to travel and socialise, gives any infection – be it the flu, Covid-19 or RSV – more room to run.

“Most public health officials would have expected an increase in Covid-19 cases, even before we knew about XBB.1.5.” said Andrew Pekosz, a professor at the Johns Hopkins Bloomberg School of Public Health who studies viral replication. “So whether the increases in Covid cases that are happening around the holidays are due to the social interactions people have had or whether they are specifically related to XBB.1.5 is still something that is not clear. Both of these things are probably contributing.

Most experts said that while they expect XBB.1.5 to have the potential to cause more disease, they don’t expect such infections to be necessarily more serious.

Looking to the Northeast, where XBB.1.5 is thought to be causing the most infections, Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota, sees cause for hope.

Osterholm notes that the upgraded boosters should provide some protection, even against this highly immune evasive strain.

“They still provide a level of immunity that may not stop you from getting infected, but it could have a significant impact on whether or not you get seriously ill and die,” he said. “I mean, right now, the most recent data we have shows that for those who have the bivalent vaccine, they have a three times lower risk of death than those who don’t.”

However, the Americans have been slow to get the new boosters. According to data from the CDC, only 15% of eligible Americans have had an upgraded booster. Among the elderly — those aged 65 and older — only about 1 in 3 have had an updated stroke.

Experts also note that while antibody treatments don’t work against this sublineage, other antivirals, such as Paxlovid and remdesivir, should still be effective.

Rapid tests continue to work, as do masks, ventilation and indoor air filtering, so even as the virus continues to evolve, there are still good ways to protect yourself from contracting Covid-19.

“It doesn’t seem to cause any more serious disease, so I think it’s a very different situation going around today than it was a year ago,” Osterholm said. “There’s so much more immunity in the population that I don’t think it’s going to take off.”

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