A highly contagious version of the Omicron variant — known officially as XBB.1.5 or by its subvariant nickname, Kraken — is quickly spreading in the U.S.

The young subvariant was first detected in New York State in the fall. It currently makes up about 28 percent of cases in the U.S. and about 72 percent of cases in the Northeast, according to the C.D.C. It’s a highly mutated version of the virus that appears able to better evade immune defenses and invade cells, with some experts calling it the most transmissible variant yet. Scientists say it remains rare in much of the world, but they expect it to spread quickly and globally.

To understand more, I spoke to my colleague Carl Zimmer, a science reporter who covers the coronavirus.

​​What should we know about XBB.1.5?

It looks like XBB.1.5 is a cut above the other Omicron subvariants in terms of getting around our defenses, and it’s also a very transmissible virus. We won’t know the full XBB.1.5 story until it’s over, but right now it’s definitely looking like it could potentially become dominant in the United States, maybe even the world.

Is it more deadly than other variants?

Unfortunately, there’s not a lot of data on its effects. From what I’ve heard from experts, it doesn’t look like it’s any more severe, which is good. But it takes time for a variant to become common enough that it infects people in large numbers, and then for some of those people to end up in the hospital, and then to analyze all those numbers. It’s really surging here in the northeastern U.S., but we don’t have great medical information systems to get quick answers on that.

Where did XBB.1.5 come from?

XBB.1.5 descends from something called XBB. And XBB is a very unusual form of Covid. It may have emerged in the spring or summer last year, possibly in India.

What’s unusual about XBB is that it was the product of two different forms of Omicron that both infected someone. As they were replicating inside that person, their genes were mixed together, and then we got a new hybrid. And this hybrid is very good at evading defenses from vaccines and infections.

So it caused a big surge in Singapore in the fall, but it didn’t really become that common elsewhere because it was competing with so many other subvariants. But as it multiplied, it started gaining more mutations. So XBB gave rise to XBB.1, and then XBB.1 mutated again into XBB.1.5. And it looks like XBB.1.5 gained a really crucial mutation that helps it grab tightly onto cells, which makes it more transmissible on top of doing a better job of escaping antibodies. So it looks really concerning. And in places like the northeastern U.S., it’s the fastest-growing variant out there.

How worried should we be?

It depends on how you were before. If you weren’t worried before, you should have been. And you should remain worried. A year ago, the first Omicron subvariant surged to dominance. Since then we have seen an explosion of new forms of Omicron. And they continue to gain mutations that help them to spread. In December, we had really fast-spreading viruses out there, and now we have one that’s even faster. So it’s a good time to take it seriously.

What will this do to the course of the pandemic in the U.S.?

I’ve been talking to scientists who are watching this really closely, and nobody thinks that we’re looking at something like what we saw a year ago, when Omicron first slammed into the U.S. Back then, we had the record-breaking caseload and record-breaking hospitalizations. It won’t be that bad, but how bad it will be, it’s hard to say. There probably will be a surge. And we’re already dealing with a lot of Covid in the U.S., so it’s a bad time for an even faster-spreading virus to show up.

How will this affect the outbreak in China?

In China, which experienced a large surge of cases in late 2022, its prospects are hard to predict. One possibility is that once other subvariants sweep through China, it will be XBB.1.5’s turn to reinfect some people there.

How protective are the new boosters against XBB.1.5?

The preliminary studies are, thankfully, looking pretty good. If you’ve been boosted, it looks like it probably reduces your chances of getting infected, and it definitely is good for keeping you out of the hospital, especially if you’re older.

Unfortunately, our rates of boosting are not great. Even if you’ve had the two shots of the primary vaccination, you’re still quite vulnerable. The updated booster can really protect a lot of people.

What’s next in the virus’s evolution?

Forecasting evolution is harder than forecasting the weather. But what’s interesting is that scientists do have some glimpses into possible futures.

The mutations that have produced XBB.1.5 have actually made it easier for this virus to gain the ability to evade more antibodies. It’s become more evolvable, you could say. So an even more evasive form of XBB.1.5 could be showing up soon.

Scientists are able to track the evolution of this virus in a way that they were never able to do in previous years with things like influenza or measles. But they still aren’t seeing things exactly in real time. So it’s possible that the next new variant is already here. It’s possible that someone walking around Rhode Island or Connecticut has already incubated the next big thing.