The BA.2 subvariant of Omicron accounts for over half of new U.S. coronavirus cases, the C.D.C. estimates.

According to estimates from the Centers for Disease Control and Prevention on Tuesday, the highly contagious Omicron subvariant known as BA.2, which led to a surge of coronavirus cases in Europe, is now the dominant version of the virus in new U.S. cases.

Last week, the World Health Organization reiterated that BA.2 was the dominant version of Omicron around the world, and Dr. Rochelle Walensky, the director of the C.D.C., said she anticipated it would become dominant in the United States in short order.

Scientists have been keeping an eye on BA.2, one of three distinct genetically distinct varieties of the Omicron variant of the coronavirus, which was discovered by South African researchers in November.

BA.2 was first identified in the United States in December, and it accounted for about 55 percent of new U.S. cases in the week ending Saturday, according to C.D.C. estimates on Tuesday. The figures are rough estimates subject to revision as more data comes in, as happened in late December, when the agency had to significantly decrease its estimate for the nationwide prevalence of the BA.1 Omicron variant. Before that, the Delta variant had been dominant since July.

Cases of Omicron can only be confirmed by genetic sequencing, which is performed on just a portion of samples across the country. The C.D.C.’s estimates vary in different parts of the country. BA.2 was found in a high proportion of samples in the Northeast, and a lower proportion of samples in the Midwest and Great Plains.

BA.1, which became dominant in late December, was almost entirely responsible for the record-shattering spike in U.S. cases this winter, but earlier this year, BA.2 started to account for a larger proportion of new infections. Its rapid growth is attributed in part to eight mutations in the gene for the spike protein on the virus’ surface, which are not found in BA.1.

While BA.2 is more transmissible than BA.1, it has not been shown to cause more severe illness and vaccines continue to protect against the worst outcomes. Many U.S. health officials have said they expect case numbers to rise without a major surge caused by BA.2, but other scientists worry that the nation isn’t doing enough to prevent another possible surge.

In the U.S., the seven-day average of new cases has dropped significantly from the height of the Omicron BA.1 surge. Though the decrease has slowed in recent days, the average has hovered this past week around 30,000 cases per day, a level last seen in July, according to a New York Times database.

Coronavirus hospitalizations plummeted in the last two weeks by about 35 percent, to about 18,000 per day. Intensive care unit hospitalizations have fallen, too — by about 42 percent, to under 3,000.

And about 750 coronavirus deaths are being reported each day in the U.S., the lowest daily average since before the Omicron variant took hold late last fall. The last time the rate was this low was in mid-August.

In some European countries, the rise of BA.2 came at the same time as a surge in new cases. In the Asia-Pacific region, Hong Kong, South Korea and New Zealand, all of which suffered relatively little from earlier variants, are now getting walloped by BA.2.

Vaccines continue to protect people against severe disease, especially those who received a booster, experts have repeatedly said. Throughout the BA.1 surge, vaccines remained highly effective against hospitalizations, and they appear to be during the BA.2 rise.