The highly contagious Omicron subvariant XBB jumped to 18% of national prevalence and accounted for more than 50% of COVID-19 cases in the Northeast, coinciding with holiday travel for millions of Americans.
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 US Centers for Disease Control and Prevention on Friday (CDC).
XBB is a subvariant of the BA.2 variant.
Top US infectious diseases expert Anthony Fauci said in November the updated boosters - 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.
BA.5 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 but are 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.
