(NewsNation) — A COVID-19 subvariant that some medical experts have dubbed as a potential “nightmare” is quickly spreading and was the dominant strain in cases reported this week.

The Centers for Disease Control and Prevention reports the omicron subvariant XBB.1.5 accounts for 40.5% of total cases in the week ending Dec. 31, overtaking the BQ.1.1 subvariant that accounted for 33.2% of cases in the week ending Dec. 24. The prevalence of XBB.1.5 has doubled since last week, when it accounted for 21.7% of cases.

The XBB subvariant is of particular concern for doctors because research has found it to be resistant to existing vaccines. However, medical experts note that vaccines continue to be an effective tool against hospitalization and severe disease.

The XBB subvariant, from which XBB.1.5 descends, is a recombinant of two subvariants that descended from the BA.2 omicron subvariant. That means it carries genetic data from two versions of the coronavirus that originated from the BA.2 subvariant.

XBB first began spreading in Asia earlier this year, and in October, doctors in New York and Boston began reporting cases of it. XBB.1.5 was most reported in the Northeast, where it accounts for roughly 75% of cases.

Scientists have found that XBB.1.5 has a higher ability to bind to cells, which may be driving its spread.

Dr. Barbara Mahon, director of the CDC’s proposed Coronavirus and Other Respiratory Viruses Division, told CBS News the agency expects the strain’s prevalence to continue to rise across the United States.

“There’s no suggestion at this point that XBB.1.5 is more severe,” Mahon said. “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.”

Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, previously told NewsNation that COVID variants will continue to crop up, but the toolkit for handling them is much larger.

“It doesn’t put us back to square one,” Adalja said in October about the emergence of XBB. “We’re not going to see the battle days of hospitals being inundated with COVID.”

Top U.S. infectious diseases expert Anthony Fauci said in November that updated COVID-19 booster shots — which target the original variant of the coronavirus as well as BA.4 and BA.5 subvariants — would still provide “some protection, but not the optimal protection” against the XBB variant.

In a study published online earlier this month in the journal Cell, researchers at Columbia University said the XBB subvariants could “further compromise the efficacy of current COVID-19 vaccines and result in a surge of breakthrough infections as well as re-infections.”

However, the researchers emphasized vaccines remain effective at preventing severe COVID cases, as well as possibly reducing the risk of long COVID.

The Hill and Reuters contributed to this report.