A masked shopper in downtown Chicago last month.Credit…Jamie Kelter Davis for The New York Times

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.

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In early March 2020, we sent the first edition of the Coronavirus Briefing newsletter, the parent of this newsletter, the Virus Briefing.

Nearly three years later, as the acute phase of the pandemic fades in the U.S., and as more of us are trying to pick up the pieces and move on, we’re winding down this newsletter. We may appear in your inbox again if the pandemic takes a drastic turn, but at the end of this month, we will pause the Virus Briefing.

Personally, it’s bittersweet. For years, I’ve waited for the day when we would happily put this newsletter to rest. In the early days, I thought that moment would come when we reached herd immunity, when we had an effective vaccine or when treatments would render the virus powerless. But over time, I think we realized that we would never experience that total release and that the virus would most likely be with us long term. While we’re certainly in a better place than when this newsletter was created, I had hoped for more closure as it came to an end.

Without a doubt, what I will miss most about this experience are the conversations with you. Over the last three years, many of you have generously opened up your lives, sharing details with us about how the virus was affecting you and your families. Thank you for trusting us with your words.

Before we go, here’s one final question, a simple one. How have three years of life during a once-in-a-generation pandemic changed you? We’re interested in the good, the bad and messy responses in between.

If you’d like to share your thoughts, you can fill out this form here. We may use your response in the final versions of our newsletter.

We recently asked readers about the lingering aftereffects of the pandemic.

I am less affected by the virus than my mother is. Ever since the news first came out about a novel virus, she has holed up in her house with boxes of gloves, masks and tests and an endless stream of CNN talking heads. When my wife and I visited, we sat on couches covered in sheets and my mother sat in another room to avoid any risks of exposure. Once we were all vaccinated, her mind eased and she removed the sheets from the couches and we could hug again. Fast-forward to late summer 2022 and my mother became infected. She had some scary side effects that luckily disappeared over a few weeks. While I believed that by finally getting the virus she would be more free from the clutches of living a hermit lifestyle, it made it worse. She barely leaves the house now except to take the trash out and complete her weekly trip to do errands. As her son, I want her to move on. I miss everything about how we were prepandemic.

— Paul M., Riva, Md.

Thanks for reading. I’ll be back next Wednesday. — Jonathan

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