New COVID Variant XBB.1.5 Now More Than A Quarter Of US Cases

The latest COVID variant of concern, XBB.1.5, continues to rise but still makes up a minority of new COVID-19 infections across the country, the Centers for Disease Control and Prevention estimated Friday, after the agency narrowed its projections of the variant's climb.

Some 27.6% of new cases nationwide are linked to the variant, the CDC currently projects, with a prediction interval ranging from 14.0 to 46.5%. The BQ.1.1 variant, which emerged in the fall, is still estimated to make up a larger share of cases for now, at 34.4%.

XBB.1.5's prevalence remains highest in the Northeast, where the CDC estimates it makes up around more than 70% of cases. In all other regions, the strain is projected to be less than a third of new infections.

Health authorities in Europe had said the CDC's initial data on the strain's growth advantage over its Omicron siblings showed it spreading faster than all other major strains seen during the pandemic so far in the U.S., except the original Omicron lineage that drove a record wave of infections last winter. 

The World Health Organization said this week it has also asked the CDC for a risk assessment of the threat posed by the new variant.

CDC officials had cautioned last week that their initial estimate of 40.5% for XBB.1.5 carried a wide margin of error. Since samples of the virus can take weeks to be sequenced, the agency uses a "Nowcast" model to predict the growth of strains.

"There is a probability interval on that. And it's really important to stress that probability interval. That's from 22.7 to 61 percent. Those intervals tend to be broader when a variant is growing in proportion quickly," Barbara Mahon, head of the CDC's proposed Coronavirus and Other Respiratory Viruses Division, told CBS News last week.

So far, federal health officials say there is no evidence that the strain itself is leading to a higher risk of severe disease compared to previous Omicron waves, based on data from where the strain was first dominant.

The current supply of updated bivalent booster shots — which target Omicron as well as the original strain — are expected to offer "some" improved protection against XBB.1.5, the Food and Drug Administration's commissioner said this week, and antiviral treatments like Pfizer's Paxlovid pills are expected to remain effective. 

XBB.1.5's ascent is displacing other Omicron variant cousins BQ.1 and BQ.1.1, which had dominated a wave of infections over the fall. Scientists believe that XBB.1.5's 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.

Most of the earliest cases from XBB.1.5 recorded in global databases through early November were sequenced around New York and Massachusetts.

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. In New England, the CDC estimates that the rate of hospitalizations among the oldest Americans is now approaching peaks seen during the winter of 2020 to 2021, but still lower than this time during the original Omicron winter.

Official case figures have not climbed substantially. However, test positivity rates suggest official tallies are steeply undercounting the spread of the virus. In places like nursing homes, where COVID-19 test results are more regularly reported to health authorities, federal data has tracked a steep rise of infections in New England.

"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 Barbara Mahon, head of the CDC's proposed Coronavirus and Other Respiratory Viruses Division.

Nearly a quarter of Americans are living in counties of "high" COVID-19 Community Levels, where the agency urges masking and other measures to curb the virus, the worst it has been since August last year. Fort Worth, Miami, and New York City rank among the most populous 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 had up only a small fraction of cases nationwide.

Beyond its parent, XBB.1.5 has an additional change called F486P. That 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. XBB appeared to be "the most profoundly resistant variant" to antibodies from breakthrough infections of any lineage tested to date.

Like BQ.1, XBB was 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 last month 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.

This is an updated version of a story first published Dec. 30, 2022.