The latest estimate released by the US Centers for Disease Control and Prevention shows nearly half of new COVID-19 cases in the country are the Omicron XBB.1.5 subvariant.
The CDC says the fast-spreading strain may account for 49.1 percent of the country's coronavirus cases for the week through Saturday.
That's an increase of about 12 percentage points from last week's figure of 37.2 percent.
The average daily number of COVID-19 hospitalizations in the US as of Tuesday was about 4,800, down nearly 20 percent from the previous week.
The average daily number of deaths as of Wednesday was around 570, topping 500 for the second week.
The World Health Organization says XBB.1.5's immune-evasive characteristics are stronger than those of other Omicron variants, and such characteristics may cause infections to increase around the world.
But it also says there is not enough data to determine if it can cause severe symptoms more easily.