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

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.

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.

BA.4 and BA.5 exhibit the qualities of escape artists, able to elude some of the antibodies produced after coronavirus vaccinations and infections, including infections caused by some earlier versions of Omicron. That may explain why these subvariants have spread even faster than others in the Omicron family. But there is not yet much evidence that they cause more severe disease.

The BA.4 and BA.5 subvariants have been detected throughout the world, and they fueled a surge of cases in South Africa in the spring, despite widespread pre-existing immunity to the virus. The wave was not as high as South Africa’s earlier waves, and deaths did not rise as sharply. Just last week, South Africa repealed its rules that required masks in indoor public spaces.

In recent weeks, more than 100,000 new coronavirus cases have been reported each day on average in the United States, according to a New York Times database, a figure that captures only a portion of the true number. Many infections go uncounted in official reports. Some scientists estimate that the current wave of cases is the second-largest of the pandemic.

As of Monday, hospitalizations in the United States were up 6 percent in the last two weeks, to an average of more than 31,000 each day, according to federal data. New deaths have stayed below 400 per day on average, data from state and local health agencies show. That is a fraction of the thousands seen daily during the winter Omicron peak.

“But in my mind, 250 deaths a day is still too many,” Dr. Rochelle Walensky, the C.D.C. director, said last week in Aspen, Colo. “The deaths that we’re seeing are generally among people who are either elderly, frail, many comorbidities who’ve had a lot of vaccine shots, or people who are unvaccinated.”

Many Americans with risk factors have said that they feel ignored and abandoned as their governments and neighbors have sought a return to normal.

As always, the spread of the virus is a regional affair. In the Northeast and Midwest, known cases have been declining for weeks, while in the South and West, cases are increasing.

Across the nation, public health rules continue to be lifted, including the ending on Saturday of an indoor mask mandate for Alameda County, the San Francisco Bay Area’s second-most populous county. In New York City, Broadway theaters — save for one — are retiring their mask requirements beginning Friday. Even the longstanding requirement to test for the coronavirus before flying to the United States from abroad was dropped this month.

While the recent availability of vaccines for children ages 6 months to 5 years was a welcome development for many parents and day care centers, experts do not expect the availability of pediatric doses to change the overall trajectory of the pandemic in the United States.

The natural waning of vaccine protection against infection over time, along with the immune evasiveness of BA.4 and BA.5, might explain why these subvariants have been able to spread quickly. It has also lent urgency to the development of Omicron-targeted boosters. While vaccine manufacturers have raced to develop these, they are based on other versions of Omicron, and it was not yet clear how well they could protect against infection with BA.4 and BA.5.

Preliminary evidence from laboratory research suggests that unvaccinated people who were infected with the version of Omicron, known as BA.1, might be easily reinfected by BA.4 or BA.5. Vaccinated people are likely to fare somewhat better, the study suggests.

But as the virus evolves, no one can tell whether retooled vaccines could become outdated by the time they become available.

“What we don’t know is what’s going to happen with a new vaccine in the fall,” Dr. Walensky said while in Aspen. “I do think we’re going to need more vaccines.”

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