Omicron subvariant BA.2 now the dominant coronavirus strain in the U.S.

(CNN) — The highly contagious Omicron subvariant BA.2 is now the dominant coronavirus strain in the United States, causing more than half of all Covid-19 infections last week, the US Centres for Disease Control and Prevention said Tuesday.

The new numbers come from the CDC’s genomic surveillance. Based on its models, the agency says that BA.2 caused between 51% and 59% of all new Covid-19 infections in the US the week ending March 26, up from an estimated 39% of all new infections the week before.

The hardest-hit region was the Northeast, where BA.2 caused more than 70% of all cases. The South and Mountain West saw the fewest cases. BA.2 caused slightly more than one-third of infections in those regions last week.

Though BA.2 is still just taking the stage in the US, it has had prominent runs in many other parts of the world, including Southeast Asia and the Western Pacific, and is winding down its European tour.

According to the World Health Organization, BA.2 is also the main cause of Covid-19 globally, out-muscling two other Omicron lineages, BA.1 and BA 1.1, to become the dominant strain. Since its takeover, international case counts — which had been declining since the first week of January — have been rising again.

In the UK, which has a more highly vaccinated population than the US, a combination of lifted restrictions, waning immunity and an even more contagious version of the virus have created a new BA.2 wave. Covid-19 cases, hospitalizations and deaths have been trending upward since the end of February, and now, the weekly average of new cases stands about where it was at the end of January.

BA.2 infections have not reached the peaks seen with BA.1, however. Case counts appear to be leveling off in the UK, though hospitalizations and deaths are still rising.

Throughout the pandemic, the US has followed the UK by about three weeks, so when cases began rising there, health officials here took notice.

In America, BA.2 has been gaining steam since the end of January, and case numbers have plateaued. That flattening conceals regional differences, however. In 13 states, weekly average numbers of new cases are rising, and they have stopped falling in 14 others, according to data collected by Johns Hopkins University.

It’s still not clear what this subvariant will do in the US. Even experts don’t exactly know what to expect.

“We’re not immune from what happens in Europe,” said former CDC Director Dr. Tom Frieden, who is now president and CEO of the nonprofit Resolve to Save Lives.

“In Europe, you see BA.2 becoming predominant and driving a resurgence, and the likelihood that will not happen in the US is pretty low, really,” Frieden said. “I do think part of the reason that we’re plateauing is that we’re about to start going up again.”

Frieden doesn’t think it’s a coincidence that the US Food and Drug Administration authorized additional booster shots for Americans who are 50 and older on Tuesday, the same day the CDC estimated that BA.2 was dominant.

But it’s anyone’s guess how high cases will go, whether lots of people will need hospital care, and whether the nation will continue to see breathtaking numbers of deaths.

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