The omicron BA.2 variant spreads about 30% more easily and has caused surges in other countries. Its steady increase in the U.S. raises questions about the wisdom of rolling back COVID restrictions.

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AILSA CHANG, HOST:

As the omicron surge continues to fade in the U.S., infectious disease experts are keeping a close eye on an even more contagious version of the virus. The fear is that this new variant could once again foil the country's hopes of quickly getting back to normal. NPR health correspondent Rob Stein reports.

ROB STEIN, BYLINE: The virus is called BA.2. It's a strain of the highly contagious omicron variant that appears to spread even more easily, and it's quickly overtaken the original omicron in South Africa and other countries. It even caused a huge second omicron surge in Denmark, so researchers like Nathan Grubaugh at Yale have been bracing for the same thing to happen in this country.

NATHAN GRUBAUGH: A lot of us were assuming that it was going to quickly take off in the United States just like it was doing in Europe and become the new dominant variant.

STEIN: But so far, that hasn't happened. Instead, according to Jeremy Luban at the University of Massachusetts, BA.2 has been slowly but steadily spreading, even as the omicron surge continued to dissipate.

JEREMY LUBAN: There are places where it's dominated pretty quickly, like Denmark. In the U.S., it's just kind of creeping up slowly.

STEIN: But the CDC says BA.2 has now been found from coast to coast and accounts for almost 4% of all new infections nationally. And Samuel Scarpino at the Rockefeller Foundation says BA.2 appears to be doubling every seven days.

SAMUEL SCARPINO: And so if it doubles again to 8%, that means that we're, you know, into the exponential growth phase, and we may be staring at another wave of COVID-19 coming in the U.S. And that's, of course, the one that we're really worried about and kind of why we're all on the edge of our seats.

STEIN: Especially because so many people are still unvaccinated in this country. And one of the remaining antibody treatments for COVID-19 looks like it may not work as well against BA.2. Now, some experts, like Grubaugh at Yale, think it's unlikely BA.2 will trigger a massive new surge because so many people have immunity from prior infections and vaccination at this point.

GRUBAUGH: The most likely thing that's going to happen is that might extend our tail, meaning it might slow down the decrease in cases, but it's probably not going to lead to a new wave of cases.

STEIN: But omicron is still infecting more than 100,000 people and killing about 2,000 people every day. So even though BA.2 doesn't appear to make people sicker than the original omicron, just slowing down the decrease will lead to a lot more suffering and death. Here's Jeremy Luban again at the University of Massachusetts.

LUBAN: There are going to be plenty of people getting sick and ending up on respirators and dying due to BA.2, so it's - when it bites, it hurts.

STEIN: And Luban says it's impossible to rule out the possibility of another surge driven by BA.2.

LUBAN: It may be that the virus has to get to somewhere like 5, 7%, and then all of a sudden it's - once it has a foothold like that, it will take off.

STEIN: That's a particular concern as mask mandates and other restrictions are being lifted around the country and people are really letting down their guard. Here's Samuel Scarpino again at the Rockefeller Foundation.

SCARPINO: There is this lurking threat of BA.2, and we need to make sure this isn't going to be a problem before we roll back all the mandates, before we tell everybody that it's safe.

STEIN: Because, Scarpino says, the nation has been blindsided before, so everyone is continuing to keep a close eye on BA.2.

Rob Stein, NPR News.

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