Another Omicron subvariant, known as BA.2.12.1, has become the dominant form among new U.S. virus cases.

Another form of the Omicron subvariant BA.2 has become the dominant version among new U.S. coronavirus cases, according to federal estimates on Tuesday, a development that experts had forecast over the last few weeks.

There was no indication yet that the new subvariant, known as BA.2.12.1, causes more severe disease than earlier forms did. BA.2.12.1 made up about 58 percent of all new U.S. cases, according to estimates by the Centers for Disease Control and Prevention for the week ending May 21.

For the first time since February, the United States recently began averaging more than 100,000 new confirmed cases per day again, according to a New York Times database. Newly reported cases have been rising in nearly every state, many infections go uncounted in official statistics, so the true number of infections may be higher. As of Monday, there were an average of more than 24,700 hospitalizations nationally, an increase of 28 percent over the last two weeks.

BA.2.12.1 spreads more rapidly than previous versions of Omicron including the form which sent U.S. cases soaring over the winter. The new version evolved from BA.2, which itself was more contagious than any variant that came before it. New York State health officials said mid-April that the Omicron subvariants known as BA.2.12 and BA.2.12.1 accounted for more than 90 percent of new cases in central New York State.

Omicron subvariants are also powering the fifth wave of virus cases in New York City, where officials put the city on “high Covid alert” last week, after rising case counts and hospitalizations reached a level that could put substantial pressure on the health care system.

But there was no sign that mask mandates were coming back in New York City, even as federal health officials warned that a large share of Americans were living in areas with “medium to high” levels of virus transmission.

And many Americans should consider wearing masks, Dr. Ashish K. Jha, the White House’s new Covid-19 coordinator said. “I feel that very strongly, that in crowded indoor spaces, in places with high transmission, people should be doing that,” he said on ABC’s “This Week” on Sunday. He also emphasized the importance of people getting vaccinated and boosted. “What we know is vaccines continue to provide a high level of protection against people getting seriously ill,” he said.

BA.2, which still makes up about 39 percent of new U.S. cases, according to the latest federal estimates, was first identified in the United States in December, and it grew to account for about 55 percent of new U.S. cases near the end of March.

Since genetic sequencing of the virus is performed on just a portion of test samples across the country, the latest C.D.C. estimates are subject to revision as more data come in. That is what happened in late December, when the agency had to significantly decrease its estimate for the nationwide prevalence of the Omicron variant known as BA.1. Before that, the Delta variant had been dominant in the United States since early summer.