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Just as the omicron surge starts to recede in parts of the U.S., scientists have their eye on another coronavirus variant spreading rapidly in parts of Asia and Europe. It's officially called "omicron BA.2," and this week scientists detected cases of it in several U.S. states, including California, Texas and Washington.

Although BA.2 is currently rare in the U.S., scientists expect it to spread in the country over the next month. There's growing evidence that it's just as contagious as — or possibly a bit more contagious than — the first omicron variant, called "omicron BA.1."

"It could be that BA.2 does have some small advantage," says 

, an epidemiologist at the University of Bern who has been tracking variants all around the world throughout the pandemic via the 

 project. "BA.2 might well be, like, 1 percent to 3 percent more transmissible, or something like that." 

So the big question now is, will that small difference be enough for this variant to lengthen the ongoing surge in the U.S., as it has in Denmark?

You can think of BA.2 as a sibling of BA.1, Hodcroft says. They share a bunch of mutations — about 30 or so — but they also have a bunch of mutations that are unique. 

"They are quite similar, but they're also different," she says. "So very much like siblings, in my opinion. Different but obviously related."

Back in November, when scientists in South Africa and Botswana discovered omicron, they didn't find just one version. They found three, called BA.1, BA.2 and BA.3 by the Phylogenetic Assignment of Named Global Outbreak Lineages at the University of Edinburgh.

The first one, BA.1, took off rapidly and spread around the world, including in the United States. And initially, it looked like BA.2 and BA.3 were weaker and less able to keep up with BA.1.

"We thought, 'OK, BA.2 is just not as fit as its sibling BA.1, and it will kind of peter out,' " Hodcroft says. 

But that's not what happened — not at all. 

Is omicron BA.2 as transmissible as omicron BA.1? 

Over the past several weeks, omicron BA.2 has begun to surprise scientists. And it's starting to look like it can, in some countries, outcompete its sibling omicron BA.1 — and, really, any other variants. 

Back in December, omicron BA.1 caused a massive surge in cases in Denmark, similar to the surge in the United States. But then, just as cases began to decline, BA.2 started spreading very rapidly in Denmark. After only a few weeks, BA.2 took over the outbreak there and has lengthened Denmark's surge. Denmark's cases are climbing steeply, with more than 40,000 recorded each day. Since the second week in January, BA.2 has caused more than 50% of those cases, 

to the Statens Serum Institut in Copenhagen.

Omicron BA.2 is also growing exponentially in 

, where it's causing at least 5% of cases in both places. Scientists are concerned it could lengthen surges in those locations as well as possibly in the United States.

Together, this data indicates that BA.2 is 

 BA.1's weaker sibling, but rather that BA.2 is quite strong and possibly more contagious. 

Many studies have shown that infections with omicron BA.1 carry a reduced risk of severe disease compared with the delta variant of the coronavirus. 

Preliminary evidence from Denmark suggests this will also be the case with omicron BA.2, says Dr. 

 at the University of California San Francisco.

"Scientists there have found that there was no increased risk in going to the hospital if you have BA.2 compared to if you have BA.1," Chin-Hong says. "That could change, but that's what we know so far." 

And there's cautious optimism about inoculations. Preliminary data from the U.K. government 

 that a third shot of a COVID-19 vaccine protects against an infection of BA.2 just as well as it does against BA.1. In both cases, it reduces the risk of a symptomatic infection by about 60 percent to 70 percent. In addition, there are many similarities between BA.1's and BA.2's spike proteins — the part of the virus that many antibodies target.

So Chin-Hong expects the vaccines will likely provide superb protection against severe disease. 

"I have no guarantee that you won't get infected or possibly reinfected [if you've already had COVID-19], meaning that you might have the sniffles or feel like you have another cold, but I feel very, very confident that you would be protected from serious disease in the general population."

And Chin-Hong says that this distinction is critical for the future of COVID-19. Going forward, he says, communities need to shift their focus from stopping all infections to keeping everyone safe from severe disease and hospitalization. 

Michaeleen Doucleff

Author

It's a sibling of the first omicron variant that swept the world. Is it more contagious? Does it cause severe disease? Will it keep current omicron surges going? Researchers are looking for answers.

A second version of omicron is spreading. Here's why scientists are on alert

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