The new omicron subvariant XBB.1.5 is seeing a “stunning” rate of increase in the U.S. and is on track to become the dominant strain circulating across the country, prompting many public health officials to voice concerns about the highly transmissible strain.

XBB.1.5, an offshoot of XBB, jumped from just 4% of COVID-19 infections in the U.S. to over 40% in the month of December after quickly surpassing the BQ subvariants.

“It is the most transmissible subvariant that has been detected yet,” Maria Van Kerkhove of the World Health Organization said at a press briefing on Wednesday.

XBB.1.5 is also spreading at a time when the Biden administration’s health officials warned of the possibility of a winter surge following gatherings over the holidays. But White House COVID-19 response coordinator Ashish Jha said the subvariant does not represent a “huge setback,” though he is concerned about it.

“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,” Jha tweeted.

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He added that whether XBB.1.5 will fuel a coronavirus wave depends on many factors, including immunity levels in the population and what actions people take to protect themselves, like masking and getting a booster shot.

Data on how the updated COVID-19 booster shots work on XBB.1.5 is limited, but Jha said that “folks without a very recent infection or a bivalent vaccine” likely have “very little protection against infection.”

For now, there is no evidence that XBB.1.5 is more severe than other omicron strains, but research is ongoing. It’s more transmissible because of the mutations it has that allow the “virus to adhere to the cell and replicate easily,” according to Van Kerkhove.

“We are concerned about its growth advantage, in particular in some countries,” Van Kerkhove said, singling out Europe and the Northeast U.S., “where XBB.1.5 has rapidly replaced other circulating variants.”

XBB.1.5 is already the dominant variant circulating in the Northeast and “will likely move to other regions quickly,” according to Robert Califf, the commissioner of the Food and Drug Administration.

“At this point, we’re experiencing an increase in cases with no evidence of increased severity of illness related to these variants,” he tweeted.

The strain is likely to challenge even the updated COVID-19 booster, which is designed to take on the original coronavirus strain and omicron subvariants BA.4 and BA.5. But Biden administration officials say they believe the shots will provide some protection.

“It is highly likely that the current bivalent vaccines provide some protection against XBB, especially in the prevention of serious illness and death,” Califf said.

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