Last week, more than 40% of Covid-19 infections in the United States were tied to an emerging omicron subvariant, known as XBB.1.5—and experts are projecting that the strain with "alarming" levels of immune evasion could drive a winter surge.

What you need to know about XBB.1.5

On Friday, CDC's Covid-19 variant dashboard featured an emerging omicron subvariant that many experts believe has the potential to trigger a new surge of Covid-19 infections.

According to GISAID, a global network that tracks Covid-19, XBB.1.5 was first detected in New York and Connecticut in late October. Initially, CDC did not include the XBB.1.5 strain in projections because it did not meet the agency's minimum threshold, said Barbara Mahon, director of the CDC's proposed Coronavirus and Other Respiratory Viruses Division.

However, the subvariant has become more prevalent in recent weeks. For the week ending on Dec. 31, CDC estimates project that the XBB.1.5 strain was responsible for 40.5% of confirmed Covid-19 cases in the United States—a significant increase from 20% for the week ending on Dec. 24. Across the Northeast, CDC estimates that the subvariant was responsible for around 75% of confirmed cases.

"For a few months now, we haven't seen a variant that's taken off at that speed," said Pavitra Roychoudhury, director of Covid-19 sequencing at the University of Washington School of Medicine's virology lab.

"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," Mahon said.

Notably, Notably, XBB.1.5 has a mutation that allows it to bind better to ACE2, the doors the virus uses to enter cells. "The mutation is clearly letting XBB.1.5 spread better," said Jesse Bloom, a computational virologist at the Fred Hutchinson Cancer Center.

"The virus needs to bind tightly to cells to be more efficient at getting in and that could help the virus be a little bit more efficient at infecting people," Andrew Pekosz, a virologist at Johns Hopkins University.

In addition, David Ho, a professor of microbiology and immunology at Columbia University, noted that XBB.1.5 exhibited "alarming" levels of immune evasion, potentially compromising the efficacy of Covid-19 vaccines. It is also resistant to all current antibody treatments.

Separately, Trevor Bedford, a professor of computational biology at the Fred Hutchinson Cancer Center said the subvariant's growth rate mirrors that of the BA.5 strain. Bedford estimates that XBB.1.5's effective reproductive number is around 1.6, which is about 40% higher than the next closest strain.

"I expect it to drive increased circulation in the coming weeks," Bedford said. According to Bedford, the increase may not be reflected in case numbers, especially since more people are testing at home, and many infections are not reported. "So I'd look to hospitalizations in the vulnerable age groups [such as seniors] as better indicator of wave," he added.

Maria Van Kerkhove, Covid-19 technical lead for the World Health Organization (WHO), said WHO is "concerned about [XBB.1.5's] growth advantage, in particular in some countries in Europe and the Northeast part of the United States, where XBB.1.5 has rapidly replaced other circulating sub-variants."

Van Kerkhove also noted that the rise in hospitalizations in the Northeast United States can't yet be attributed to XBB.1.5 because other illnesses, including the flu, could also be responsible.

Experts are still learning about XBB.1.5

Ultimately, many experts have noted that it is difficult to determine how much of the subvariant's growth can be attributed to its properties and how much can be attributed to timing. During the holiday season, increased travel and social interactions can increase the spread of any virus.

"Most public health officials would have expected an increase in Covid-19 cases, even before we knew about XBB.1.5." Pekosz said. "So whether the increases in Covid cases that are occurring during the holidays are occurring because of the social interactions that people have had or whether they're specifically related to XBB.1.5 is still something that isn't clear. Both of those things are probably contributing."

While many experts believe XBB.1.5 has the potential to trigger a surge of Covid-19 cases, they do not necessarily expect the infections to be more severe. "There's no suggestion at this point that XBB.1.5 is more severe," Mahon said.

In addition, John Moore, a professor of microbiology and immunology at Weill Cornell Medical College, said he expects cases to peak by mid-January.

According to Michael Osterholm, who directs the University of Minnesota's Center for Infectious Disease Research and Policy, updated boosters should still provide some protection against the evasive XBB.1.5 strain.

"They still provide a level of immunity that may not prevent you from getting infected but may have a significant impact on whether or not you become seriously ill and die," Osterholm said. "I mean, right now, the most recent data we have shows that for those who have the bivalent vaccine, they have a three-fold lower risk of dying than those who don't."

White House Covid-19 response coordinator Ashish Jha said in a tweet on Wednesday that, while XBB.1.5 has had a "stunning increase," it's not yet clear whether it's more dangerous than previous mutations.

"If you had an infection before July or your last vaccine was before bivalent update in September, your protection against an XBB.1.5 infection is probably not that great," he said. However, Jha added that Covid-19 antivirals like Paxlovid and Molnupiravir should still work in treating infections caused by XBB.1.5.

Similarly, FDA Commissioner Robert Califf said it's "highly likely that the current bivalent vaccines provide some protection against XBB, especially in the prevention of serious illness and death." (Scribner, Axios, 12/31; Goodman, CNN, 1/3; Paun, Politico, 1/4; Choi, The Hill, 1/4)