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People wonder if they should keep calm and carry on in the face of delta plus variant

SARAH MCCAMMON, HOST:

A new coronavirus variant is raising concern in the U.K. Some are calling it delta plus. There's worry that it could be more dangerous than the current version of delta circulating in the United States. To fill us in on what's known about this new mutant, we've brought in NPR's variant expert, global health correspondent Michaeleen Doucleff. Hey there, Michaeleen.

MICHAELEEN DOUCLEFF, BYLINE: Hi, Sarah.

MCCAMMON: OK. The name of the variant sounds a little scary, right? Delta plus - what does that mean? Is it related to delta, first of all?

DOUCLEFF: Yes, it is related to delta. And here's how. So, after delta emerged months ago, it continued to mutate and pick up mutations. And these mutations created many, many variants of delta all over the world, even here. And you can kind of think of them like grandchildren of delta. They're all descendants of delta. I talked to Bill Hanage at Harvard about this. He's an epidemiologist. He says over the past few months, the media has called many of these delta variants one thing - delta plus.

BILL HANAGE: Delta plus has been used interchangeably for any descendant of delta that people are getting their knickers in a twist about. Now, whether they should or should not be getting their knickers in a twist is another question.

DOUCLEFF: Because, Hanage says, in the past, all these previous delta pluses have turned out not to be really any different than delta. They aren't more dangerous or more transmissible.

MCCAMMON: OK. And what about this new delta plus that's spreading in the U.K.?

DOUCLEFF: Yes. It's officially known as AY.4.2.

MCCAMMON: OK. It accounts now for more than 6% of the cases in the U.K. Does that mean it's more transmissible than the current version of delta that's dominating here in the U.S.?

DOUCLEFF: So scientists can't answer that for sure because there isn't enough data yet. But that said, given how the variant is behaving in the U.K., Hanage estimates that it's likely to be just a tiny bit more transmissible than the original delta - like, just 10% more. And remember. Delta is already super, super contagious. So adding 10% more probably won't change the behavior very much. It's like, if you have a supercharged engine, and you bump up the turbo maybe 10% more, it's still just supercharged.

MCCAMMON: OK. That makes sense. But it's still an increase in transmissibility. So could delta plus sweep in and reverse this lull in cases that we've been seeing here in the U.S., the way that the regular delta variant did over the summer?

DOUCLEFF: I asked a bunch of scientists this question. And one of them is Jeremy Luban at the University of Massachusetts Medical School.

JEREMY LUBAN: There's no obvious sign that it's going to take off the way that delta did, like a totally new thing that just takes over. It's more like, you know, maybe a kind of slowly creeping.

DOUCLEFF: So every scientist I spoke with agreed with that. And one of them has even used computer models to see what might happen when new variants come to the U.S. His name is Justin Lessler. He's at the University of North Carolina. He ran a simulation with a variant that's 50% more transmissible than delta and then looked for a big surge.

JUSTIN LESSLER: It didn't get back to the types of peaks we saw last winter, or even the types of peaks we've seen in the delta wave.

MCCAMMON: Good to hear. But why is that the case? Why wouldn't something that's super transmissible get us in trouble?

DOUCLEFF: Yeah, it's an interesting question. So Lessler says it's largely because, right now, the U.S. population is protected from SARS-CoV-2. So many people have been exposed through infections or vaccines or both of them, there's a high level of immunity across our population.

MCCAMMON: NPR Global Health Correspondent Michaeleen Doucleff. Thank you so much, Michaeleen.

DOUCLEFF: Thank you, Sarah. Transcript provided by NPR, Copyright NPR.