CDC tracking rise of new XBB.1.5 COVID variant, now dominant strain among U.S. cases
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The Centers for Disease Control and Prevention said Friday it is tracking a new variant of concern dubbed XBB.1.5. According to figures published last week, it estimates XBB.1.5 makes up 40.5% of new infections across the country — making it the dominant strain driving new U.S. infections.
XBB.1.5’s ascent has overtaken other Omicron variant cousins BQ.1 and BQ.1.1, which had dominated a wave of infections over the fall. Scientists believe its recent growth could be driven by key mutations on top of what was already one of the more immune evasive strains of Omicron to date.
“We’re projecting that it’s going to be the dominant variant in the Northeast region of the country and that it’s going to increase in all regions of the country,” said Dr. Barbara Mahon, director of the CDC’s proposed Coronavirus and Other Respiratory Viruses Division, in an interview with CBS News.
Mahon said the agency had not listed XBB.1.5 separately in its earlier projections because the strain had not cleared a minimum threshold in the underlying sequences collected by the agency.
The agency’s 40.5% figure is only a projection, Mahon stressed, with a probability interval ranging right now from 22.7% to 61.0%.
XBB.1.5’s prevalence is largest in the Northeast, the agency estimates. Most of the earliest cases from XBB.1.5 recorded in global databases through early November were sequenced around New York and Massachusetts.
More than 70% of infections in the regions spanning New Jersey through New England are now from XBB.1.5, the agency is projecting.
An increase in hospitalizations
The ascent of XBB.1.5 comes as COVID-19 hospitalizations have accelerated across the U.S. in recent weeks. The pace of new admissions is now worse than this past summer’s peak in several regions, but still lower than at this time last winter.
“There’s no suggestion at this point that XBB.1.5 is more severe. But I think it is a really good time for people to do the things that we have been saying for quite a while are the best ways to protect themselves,” said Mahon.
This month, the Northeast has recorded some of the worst COVID-19 hospital admission rates out of any region in the country. In New England, the CDC says new hospitalizations among Americans 70 and older have climbed to the highest levels seen since early February.
“It’s by far the most infectious variant we’ve ever had, so much so that it was 4% of cases beginning of December, and by end of December, it’s 40% of cases,” said CBS News medical contributor Dr. David Agus.
But there is no data that people are getting sicker, he said, or that XBB.1.5 is evading vaccines.
Around 13% of Americans are currently living in areas of “high” COVID-19 Community Levels, where the agency currently urges masking. Los Angeles, Miami, and New York City rank among the biggest counties now at these levels.
Mahon said XBB.1.5’s mutations could be part of driving the increase where XBB had failed to gain a foothold. But she added that other factors, like the higher risk posed by respiratory viruses during the winter holiday season, could also be playing a factor.
Mahon cited the agency’s recommendations to seek out updated COVID booster shots, as well as taking other precautions like improving ventilation, testing before gathering, or masking in high COVID areas.
“So that advice doesn’t change at all. And this time of year is a really good time to be following that advice,” said Mahon.
Ungrouping XBB.1.5 from XBB
The XBB.1.5 strain is a spinoff of the XBB variant, itself a “recombinant” blend of two prior Omicron strains, which drove a wave of infections overseas earlier this year.
Earlier this year, the Biden administration had voiced optimism that XBB was unlikely to dominate infections in the country. South Asian nations like Singapore reported that strain appeared to pose a lower risk of hospitalization relative to earlier Omicron variants.
After it was first spotted in the country, XBB had made up a small fraction of U.S. cases for several weeks despite appearing in a growing share of variants from arriving international travelers.
Then over the past month, XBB’s prevalence began to swell in the CDC’s estimates. These figures are released weekly in “Nowcast” projections based on the sequences that authorities have gathered so far.
Now, the CDC says that increase was driven largely by XBB.1.5. After ungrouping XBB.1.5, the agency estimates all other XBB infections currently make up just 3% of cases nationwide.
Beyond its parent, XBB.1.5 has an additional change called S486P. Chinese scientists have reported the mutation appears to offer a “greatly enhanced” ability to bind to cells, which could be helping drive its spread.
“We’ve been tracking XBB for weeks as I said, and it was XBB and XBB.1, and they really weren’t taking off. They weren’t increasing rapidly in proportion,” said Mahon.
Vaccines, treatments, and tests
Before evolving into XBB.1.5, XBB had already ranked among the strains with the largest immune-evasion relative to earlier major Omicron strains. Scientists in Japan reported this week that XBB appeared to be “the most profoundly resistant variant” to antibodies from breakthrough infections of any lineage they had tested.
Like BQ.1, XBB is resistant to a roster of monoclonal antibody drugs that doctors had relied on earlier in the pandemic before they were sidelined by new variants.
Data from a team of federally-backed researchers earlier this year found the current batch of updated bivalent boosters appear to offer better “neutralizing activity” Omicron variants, including XBB, when testing antibodies in the blood of people who got the updated booster compared to after only the original vaccines.
“We expect that the bivalent booster will provide protection against XBB.1.5 as it has against other Omicron subvariants. And if people haven’t gotten it yet, this is a great time to do it,” Mahon said.
However, antibody responses in that study were also worse for XBB compared to the other strains they studied.
“The XBB.1.5 variant would look similar to the XBB we tested in our study. The R346T/I mutation within the spike increases the ability of the virus to evade antibodies more efficiently,” Emory University’s Mehul Suthar told CBS News in an email.
For antiviral drugs like Pfizer’s Paxlovid, data from another team of scientists in Japan suggest they will retain efficacy against XBB.
“With what we know so far, XBB.1.5 has not acquired any new mutations in the viral protein targeted by Paxlovid. The susceptibility of XBB.1.5 against Paxlovid should not change given the current data,” the University of Wisconsin-Madison’s Peter Halfmann, one of that study’s authors, told CBS News in an email.
And for tests, the Food and Drug Administration warned Thursday on its website that one home collection kit — DxTerity’s saliva test for the virus — had been discovered to have “significantly reduced sensitivity” to variants with XBB’s mutations.
“We will update the page when significant new information becomes available, including when the FDA’s analyses identify tests for which performance may be impacted for known SARS-CoV-2 variants,” Jim McKinney, a spokesperson for the regulator, said in a statement.
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