(Reuters) - The highly-contagious Omicron subvariant XBB has surged to more than 50% of COVID-19 cases in the northeastern United States and risks spreading fast as millions of Americans begin holiday travel on Friday.
In the week ended Dec. 24, XBB was estimated to account for 18.3% of the COVID-19 cases in the United States, up from 11.2% in the previous week, according to the U.S. Centers for Disease Control and Prevention on Friday (CDC).
The subvariant is currently dominant in the Northeast, but accounts for fewer than 10% of infections in many other parts of the country, the CDC said.
Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, said holiday travel in the United States could speed up the XBB subvariant's spread across the country.
The American Automobile Association (AAA) had estimated that 112.7 million people planned to travel 50 miles (80 km) or more from home between Friday and Jan. 2, up 3.6 million travelers over last year and closing in on pre-pandemic numbers.
But that number was likely to be diminished by the treacherous weather complicating air and road travel going into the weekend.
"Anytime a new variant moves to a different geographic area, it does run the risk of sort of spawning a mini-outbreak in that area," Pekosz said.
Still, Pekosz said he does not see the XBB subvariant driving the kind of massive surges seen last winter from the original Omicron variant.
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.
XBB is a subvariant of the BA.2 variant.
The earliest BA.5 lineage now represents just a small fraction of cases, having been overtaken by its offshoots, BQ.1 and BQ.1.1, which still remain the dominant variants in the United States, though on the decline.
The rise in cases of the new variant comes a week after the White House COVID response coordinator urged Americans to get their flu vaccines and updated COVID-19 boosters, pointing to rising cases in about 90% of the country ahead of the year-end holidays.
The XBB variant has been driving up cases in parts of Asia, including Singapore. While some experts have said it is more transmissible, it has not resulted in a surge in hospitalizations.
BQ.1.1 and BQ.1 are expected to account for 63.1% of cases in the United States, compared with 64.6% a week ago, the CDC said.