NEW YORK (NYTIMES) - Another form of the Omicron subvariant BA.2 has become the dominant version among new US coronavirus cases, according to federal estimates released Tuesday (May 24), a development that experts had forecast over the past 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 per cent of all new US cases, according to estimates by the Centres for Disease Control and Prevention for the week ending May 21.

As Americans approach their third Memorial Day weekend of the pandemic, the country is averaging more than 100,000 new confirmed cases per day for the first time since February, 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 people with the virus hospitalised nationally, an increase of 28 per cent over the past two weeks.

BA.2.12.1 spreads more rapidly than previous versions of Omicron including the form which sent US 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 in mid-April that the Omicron subvariants known as BA.2.12 and BA.2.12.1 accounted for more than 90 per cent of new cases in central New York state.

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

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 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.

Last month, a federal judge in Florida struck down a mask mandate on public transport and the country's largest airlines stopped requiring masks on flights, ending a practice that most carriers followed for nearly two years.

Dr Jha also emphasised 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.

In a sign of growing concern, the CDC said late last week that all people 50 or older should get a second booster shot if at least four months have passed since their first booster, strengthening its recommendation.

Still, conditions appear to be stabilising in some north-eastern states that were among the first to see a spring surge in cases. Though still high, case rates have started to level off or decline in New Jersey, New York and Rhode Island.

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