Omicron subvaniants BA.4 and BA.5 have become dominant in the U.S.--CDC

The Omicron subvariants BA.4 and BA.5 have together become dominant in the U.S., the C.D.C. estimates

Continuing their rapid march across the United States, the Omicron subvariants known as BA.4 and BA.5 have together become dominant among new coronavirus cases, according to new estimates on Tuesday from the Centers for Disease Control and Prevention.

As of the week ending Saturday, BA.4 made up 15.7 percent of new cases, and BA.5 was 36.6 percent, accounting for about 52 percent of new cases in the United States, numbers that experts said should rise in the weeks to come.

The statistics, released Tuesday morning, are based on modeling and can be revised as more data comes in, which happened in late December, when the agency’s estimates missed the mark.

The release came on the day that an expert committee recommended Tuesday that the Food and Drug Administration plan on an updated booster shot of the coronavirus vaccines that targets some form of the Omicron variant, even though the virus might evolve yet again by the fall.

In the less than six months since BA.4 and BA.5 were first detected in South Africa, the two subvariants appeared in the United States to be overtaking two earlier Omicron subvariants, including BA.2, which was the prevailing version for a time earlier this spring. The other, BA.2.12.1, was dominant alone as of the week ending June 18, according to C.D.C. estimates. Over the winter, the form of Omicron that first emerged in the United States sent case counts soaring. Before that, the Delta variant had been dominant in the United States since early summer.