麻豆传媒

The new Omicron subvariant XBB.1.5 is growing rapidly in the U.S., becoming one of the most common strains along with BQ.1.1 and BQ.1.

XBB.1.5 now accounts for an estimated 18.3% of COVID cases in the U.S., up from 11.5% the week prior, . [Editors' Note: And up to 27.6% for the week ending January 7.]

Just 1 month ago, in the first week of December, XBB.1.5 accounted for only 2% of all COVID cases in the U.S., CDC data show.

It's particularly prevalent in the Northeast, found in about 60% of infections there, according to the agency.

Overall in the U.S., the BA.5 variant has all but disappeared, while BQ.1.1 is the second most common variant, at 37% of cases, followed by BQ.1 at 24%.

XBB.1.5 evolved from XBB.1, which evolved from XBB -- an Omicron subvariant that emerged in India in mid-August and quickly became predominant there, as well as in Singapore and other regions in Asia, .

XBB involved a recombination of two descendants of the BA.2 variant, , of Fred Hutchinson Cancer Center in Seattle.

Notably, in Singapore, XBB was responsible for a surge in cases, but it didn't cause an increase in hospitalizations or death, Ali Mokdad, PhD, of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in Seattle, previously told 麻豆传媒.

Currently there's no indication that XBB.1.5 causes more severe illness than other Omicron subvariants, as there hasn't been a greater increase in hospitalizations in regions hit hardest by the strain.

Still, it remains to be seen whether this variant will drive an overall surge in cases given its increased transmissibility, Bloom said.

Bloom said that while XBB.1.5 is equally as immune evasive as the two other XBB lineages -- which had a significant amount of antibody escape, according to -- it has a higher affinity for ACE2, which explains its increased transmissibility.

Driving that higher affinity for ACE2 is a change at site 486, which has been a "major site of antibody escape going back to the earliest variants," .

"It's easy to understand why it took longer for variants to emerge at site 486: mutations at 486 reduce ACE2 affinity, so benefit they provide in antibody escape comes at cost to receptor binding," . " between XBB.1.5 and its immediate parent XBB.1 is that it has traded the more costly F486S mutation for F486P. Therefore, XBB.1.5 isn't expected to have more antibody escape than XBB.1 (which already had mutated F486), but it should have greater ACE2 affinity."

Correction: This story was updated to reflect revised numbers from the CDC.

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