CDC tracks rapid increase of new XBB.1.5 COVID variant, not yet a majority nationwide

The newest COVID variant of concern, XBB.1.5, continues to rise but still accounts for a minority of new COVID-19 infections nationwide, the Centers for Disease Control and Prevention estimated Friday, after the agency lowered its estimates for the variant’s rise .

About 27.6% of new cases nationwide are linked to the variant, the CDC currently estimates, with a prediction range from 14.0 to 46.5%. the BQ.1.1 variant, that appeared autumn, is still estimated to make up a larger proportion of the cases for now, with 34.4%.

XBB.1.5’s prevalence remains highest in the Northeast, where the CDC estimates it accounts for about 70% of cases. In all other regions, the strain is estimated to account for 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 spread faster than any other major strain seen during the pandemic so far in the United States, except for the original Omicron lineage that fueled a record wave of infections last winter.

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

CDC officials had warned last week that their initial estimate of 40.5% for XBB.1.5 had 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’s a probability interval on that. And it’s very important to emphasize that probability interval. It’s from 22.7 to 61 percent. Those intervals tend to be wider when a variant grows rapidly in proportion,” suggests Barbara Mahon, head of the CDC. 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 leads to a higher risk of serious 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 targets Omicron as well as the original strain — is expected to offer “some” improved protection against XBB.1.5, the Food and Drug Administration’s commissioner said this week, and antiviral treatments such as Pfizer’s Paxlovid pills are expected to remain effective.

The XBB.1.5’s ascent displaces other Omicron variants BQ.1 and BQ.1.1, which had dominated a wave of infections throughout the autumn. Researchers believe that XBB.1.5’s recent growth may 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 XBB.1.5 cases recorded in global databases through early November were sequenced around New York and Massachusetts.

Increase in hospitalizations

The ascent to XBB.1.5 comes as COVID-19 hospitalizations have accelerated across the United States in recent weeks.

The pace of new admissions is now worse than last summer’s peak in several regions. In New England, the CDC estimates that hospitalizations among older Americans are now approaching the peaks seen during the winter of 2020 to 2021, but still lower than this time during the original Omicron winter.

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

“There is currently no indication that XBB.1.5 is more serious. But I think it is a very good time for people to do the things that we have long said are the best ways to protect themselves,” said Barbara Mahon, head of the CDC’s proposed division of coronaviruses and other respiratory viruses.

Nearly a quarter of Americans live in counties with “high” community levels of COVID-19, where the agency is urging masking and other measures to contain the virus, the worst since last August. Fort Worth, Miami and New York City are among the most populous counties now at these levels.

Mahon said XBB.1.5’s mutations may be part of driving the increase where XBB had failed to gain a foothold. But she added that other factors, such as the higher risk of respiratory viruses during the winter holidays, may also play a factor.

Mahon cited the agency’s recommendations to seek out updated COVID booster shots, in addition to taking other precautions such as improving ventilation, testing before collection or masking in areas with high rates of COVID-19.

“So that advice doesn’t change at all. And this time of year is a very good time to follow that advice,” Mahon said.

Splits XBB.1.5 from XBB

The XBB.1.5 strain is a spinoff of the XBB variant, itself a “recombinant” mix of two earlier Omicron strains, which led to a wave of infections abroad earlier this year.

Earlier this year, the Biden administration had expressed optimism that XBB was unlikely to dominate infections in the country. South Asian nations such as Singapore reported that strain appeared to pose a lower risk of hospitalization compared to earlier Omicron variants.

After it was first detected in the country, XBB had made up a small fraction of US cases for several weeks, despite showing up in a growing proportion of variants from arriving international travelers.

So over the last month, XBB’s prevalence began to swell in the CDC’s estimates. These numbers are published weekly in “Nowcast” projections based on the sequences that the authorities have collected so far.

Now the CDC says the increase was largely driven by XBB.1.5. After splitting out XBB.1.5, the agency estimates that all other XBB infections accounted for only a small fraction of cases nationwide.

Beyond the overall, XBB.1.5 has an additional change called F486P. That mutation seems to offer one “greatly improved” ability to bind to cells, which can help drive the spread.

“We’ve been tracking XBB for weeks like I said, and it was XBB and XBB.1, and they really didn’t take off. They didn’t increase rapidly in proportion,” Mahon said.

Vaccines, treatments and tests

Before XBB evolved into XBB.1.5, XBB had already ranked among the most immune evasive strains of previous major Omicron strains. XBB seemed to be “the most resistant variety” against antibodies from breakthrough infections of any lineage tested to date.

Like BQ.1, XBB was resistant to a list of monoclonal antibody drugs that doctors had relied on earlier in the pandemic before being sidelined by new variants.

Data from a team of federally supported researchers earlier this year found that the current batch of updated bivalent boosters appear to offer better “neutralizing activity” to Omicron variants, including XBB, when testing antibodies in the blood of people who received the updated booster compared to after only the original vaccines.

“We expect the bivalent booster to provide protection against XBB.1.5 like it does against other Omicron subvariants. And if people haven’t gotten it yet, this is a great time to do so,” Mahon said.

However, the antibody responses in that study were also worse for XBB compared to the other strains they studied.

“The XBB.1.5 variant will be similar to the XBB we tested in our study. The R346T/I mutation in the spike increases the virus’ ability to evade antibodies more effectively,” Emory University’s Mehul Suthar told CBS News in an email.

For antiviral drugs such as Pfizer’s Paxlovid, data from another team of researchers in Japan suggest they will retain their effectiveness against XBB.

“With what we know so far, XBB.1.5 has not acquired any new mutations in the viral protein that Paxlovid targets. The sensitivity of XBB.1.5 to Paxlovid should not change given the current data,” University of Wisconsin-Madison’s Peter Halfmann , one of the authors of that study, 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 found to have “significantly reduced sensitivity” for variants with XBB’s mutations.

“We will update the page when important new information becomes available, including when FDA’s analyzes identify tests that may affect performance for known SARS-CoV-2 variants,” Jim McKinney, a spokesman for the regulator, said in a statement.

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

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