(CNN) — For weeks, scientists have been watching as a series of descendants of the omicron variant vied for dominance of the transmission of covid-19 in the United States. The BQ -BQ.1 and BQ.1.1 seemed to overpower all the others to claim a slight advantage.
The result has been a gradual rise in cases and hospitalizations that never seemed to reach the high points of this summer’s BA.5 wave, and was certainly nothing like the tsunami of disease triggered by the original omicron a year ago. .
But on Friday, the US Centers for Disease Control and Prevention’s (CDC) Covid-19 Variant Panel revealed a new player that could soon take the world by storm: XBB.1.5.
The CDC estimates that the XBB.1.5 variant has more than doubled its share of covid-19 distribution every week for the last four, going from 4% to 41% of new infections during December. In the Northeast, the CDC estimates that XBB.1.5 is causing 75% of new cases.
“For a few months, we haven’t seen a variant take off at that rate,” said Pavitra Roychoudhury, director of Covid-19 sequencing in the Virology Laboratory at the University of Washington School of Medicine.
Virologists and epidemiologists say that this omicron subvariant has characteristics that give it the potential to fuel a new wave of covid-19 cases in the US, although it is not yet clear how big that wave will be and whether it could send many more people to the hospital.
Despite recent concerns that a new covid-19 threat could stem from the current surge of cases in China, experts note that XBB.1.5 appears to have emerged in the United States. It was first detected in New York and Connecticut in late October, according to GISAID, a global initiative to catalog and track variants of the coronavirus.
Trevor Bedford, professor of computational biology at the Fred Hutchinson Cancer Center in Seattle, said that XBB.1.5 has a similar growth rate to its distant cousin BA.5.
Bedford has put its effective reproductive number – the number of new infections each infected person is expected to cause – at around 1.6, about 40% more than its next closest competitor.
“I anticipate it will drive increased circulation in the coming weeks,” Bedford wrote in an email. That increase may not be reflected in the number of cases, she noted, as more and more people are getting tested at home, and their cases may go undercounted unless they seek medical attention and get a lab test to confirm their results. . “So I would look at hospitalizations in vulnerable age groups (such as the elderly) as the best indicator of the surge,” she wrote.
XBB.1.5 is the product of recombination: two offspring of BA.2, the subvariant that fueled a modest surge of cases in the US in April, swapped parts of their genetic code, resulting in 14 new mutations in the virus spike proteins compared to BA.2, and to a new subvariant, XBB.
XBB sparked a surge of cases in Singapore this fall, but failed to gain much ground in the United States. Here, it had to compete against a bunch of co-circulating variants that had independently evolved some of the same mutations, making them more alike.
Even so, scientists have been keeping a close eye on XBB and its variations.
Dr. David Ho, Professor of Microbiology and Immunology at Columbia University, recently tested viruses designed to have the spikes of XBB and XBB.1, as well as BQ.1 and BQ 1.1, in his lab against antibodies in the blood of 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 subvariants.
He found that XBB.1 was the most elusive 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 compared with BA.4 and BA.5.
In terms of immune evasion, Ho says, these variants have strayed as far from the antibodies we’ve made to use against them as the original omicron variant strayed from the covid-19 variants that preceded it by about a year ago.
Dr Ho calls these levels of immune evasion “alarming” and says they could further compromise the efficacy of Covid-19 vaccines. His findings were recently published in the journal Cell.
Ho explained Monday that XBB.1.5’s antibody evasion is the same as that of XBB.1, meaning it has the potential to escape protection from vaccines and previous infections. It is also resistant to all current antibody treatments, including Evusheld.
Another ace up the sleeve of XBB.1.5
In addition to its strong immune evasion capabilities, XBB.1.5 has another ace up its sleeve that seems to be contributing to its growth. It has a key mutation at the 486 site, which allows it to bind more tightly to ACE2, the gate the virus uses to enter our cells.
“The mutation clearly allows XBB.1.5 to spread better,” Jesse Bloom, a computational virologist at the Fred Hutchinson Cancer Center, wrote in an email.
This mutation was first pointed out by Bloom, who studies the evolution of viruses and viral proteins, as one that might be important for viral fitness. Yunlong Cao of Peking University confirmed it.
“It has more ability to enter cells,” says Roychoudhury, which means it’s more infectious.
Still, experts say it’s hard to know how much of XBB.1.5’s growth can be attributed to the virus’s properties and how much to just good timing.
The arrival of the holidays, during which people are more likely to travel and socialize, gives any infection – be it flu, covid-19 or RSV – more room to spread.
“Most public health officials would have expected an increase in COVID-19 cases, even before we knew about XBB.1.5,” says Andrew Pekosz, a professor at the Johns Hopkins Bloomberg School of Public Health. , who studies viral replication. “So whether the increase in covid cases that is occurring over the holidays is due to the social interactions people have been having or is related specifically to XBB.1.5 is still unclear. Both of these are probably contributing “.
Most experts said that while they expect XBB.1.5 to have the potential to cause more illness, they don’t expect those infections to be necessarily more serious.
Looking to the northeast, where XBB.1.5 is believed to be causing the most infections, Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota, sees reason for hope.
Osterholm notes that the updated booster doses should provide some protection, even against this immunologically elusive variant.
“They still provide a level of immunity that may not prevent you from getting infected, but can have a significant impact on whether or not you get seriously ill and die,” he said. “Right now, the most recent data we have shows that those who have the bivalent vaccine have a three times lower risk of dying than those who do not.”
However, Americans have been slow to get vaccinated. According to CDC data, only 15% of eligible Americans have gotten the updated booster shot. Among those over 65, only 1 in 3 have been vaccinated.
The experts also note that while antibody treatments will not work against this subvariant, other antivirals, such as Paxlovid and remdesivir, should still be effective.
Rapid tests still work, as do masks and indoor air ventilation and filtration, so even as the virus continues to evolve, there are still good ways to protect yourself from getting Covid-19.
“It doesn’t appear to be causing more serious disease, so I think a very different situation is going on today than it was a year ago,” Osterholm says. “There is increased immunity in the population, so I don’t think it’s going to break out.”