How Worried Should We Be About XBB.1.5?

After months and months of SARS-CoV-2 subvariant soup, one ingredient has emerged in the United States with a flavor pungent enough to overwhelm the rest: XBB.1.5, an Omicron offshoot that now accounts for an estimated 75 percent of cases in the Northeast. A crafty dodger of antibodies that is able to grip extra tightly onto the surface of our cells, XBB.1.5 is now officially the country’s fastest-spreading coronavirus subvariant. In the last week of December alone, it zoomed from 20 percent of estimated infections nationwide to 40 percent; soon, it’s expected to be all that’s left, or at least very close. “That’s the big thing everybody looks for—how quickly it takes over from existing variants,” says Shaun Truelove, an infectious-disease modeler at Johns Hopkins University. “And that’s a really quick rise.”

All of this raises familiar worries: more illness, more long COVID, more hospitalizations, more health-care system strain. With holiday cheer and chilly temperatures crowding people indoors, and the uptake of bivalent vaccines at an abysmal low, a winter wave was already brewing in the U.S. The impending dominance of an especially speedy, immune-evasive variant, Truelove told me, could ratchet up that swell.

But the American public has heard that warning many, many, many times before—and by and large, the situation has not changed. The world has come a long way since early 2020, when it lacked vaccines and drugs to combat the coronavirus; now, with immunity from shots and past infections slathered across the planet—porous and uneven though that layer may be—the population is no longer nearly so vulnerable to COVID’s worst effects. Nor is XBB.1.5 a doomsday-caliber threat. So far, no evidence suggests that the subvariant is inherently more severe than its predecessors. When its close sibling, XBB, swamped Singapore a few months ago, pushing case counts up, hospitalizations didn’t undergo a disproportionately massive spike (though XBB.1.5 is more transmissible, and the U.S. is less well vaccinated). Compared with the original Omicron surge that pummeled the nation this time last year, “I think there’s less to be worried about,” especially for people who are up to date on their vaccines, says Mehul Suthar, a viral immunologist at Emory University who’s been studying how the immune system reacts to new variants. “My previous exposures are probably going to help against any XBB infection I have.”

SARS-CoV-2’s evolution is still worth tracking closely through genomic surveillance—which is only getting harder as testing efforts continue to be pared back. But “variants mean something a little different now for most of the world than they did earlier in the pandemic,” says Emma Hodcroft, a molecular epidemiologist at the University of Bern, in Switzerland, who’s been tracking the proportions of SARS-Cov-2 variants around the world. Versions of the virus that can elude a subset of our immune defenses are, after all, going to keep on coming, for as long as SARS-CoV-2 is with us—likely forever, as my colleague Sarah Zhang has written. It’s the classic host-pathogen arms race: Viruses infect us; our bodies, hoping to avoid a similarly severe reinfection, build up defenses, goading the invader into modifying its features so it can infiltrate us anew.

But the virus is not evolving toward the point where it’s unstoppable; it’s only switching up its fencing stance to sidestep our latest parries as we do the same for it. A version of the virus that succeeds in one place may flop in another, depending on the context: local vaccination and infection histories, for instance, or how many elderly and immunocompromised individuals are around, and the degree to which everyone avoids trading public air. With the world’s immune landscape now so uneven, “it’s getting harder for the virus to do that synchronized wave that Omicron did this time last year,” says Verity Hill, an evolutionary virologist at Yale. It will keep trying to creep around our defenses, says Pavitra Roychoudhury, who’s monitoring SARS-CoV-2 variants at the University of Washington, but “I don’t think we need to have alarm-bell emojis for every variant that comes out.”

Some particularly worrying variants and subvariants will continue to arise, with telltale signs, Roychoudhury told me: a steep increase in wastewater surveillance, followed by a catastrophic climb in hospitalizations; a superfast takeover that kicks other coronavirus strains off the stage in a matter of days or weeks. Omens such as these hint at a variant that’s probably so good at circumventing existing immune defenses that it will easily sicken just about everyone again—and cause enough illness overall that a large number of cases turn severe. Also possible is a future variant that is inherently more virulent, adding risk to every new case. In extreme versions of these scenarios, tests, treatments, and masks might need to come back into mass use; researchers may need to concoct a new vaccine recipe at an accelerated pace. But that’s a threshold that most variations of SARS-CoV-2 will not clear—including, it seems so far, XBB.1.5. Right now, Hodcroft told me, “it’s hard to imagine that anything we’ve been seeing in the last few months would really cause a rush to do a vaccine update,” or anything else similarly extreme. “We don’t make a new flu vaccine every time we see a new variant, and we see those all through the year.” Our current crop of BA.5-focused shots is not a great match for XBB.1.5, as Suthar and his colleagues have found, at least on the antibody front. But antibodies aren’t the only defenses at play—and Suthar told me it’s still far better to have the new vaccine than not.

In the U.S., wastewater counts and hospitalizations are ticking upward, and XBB.1.5 is quickly elbowing out its peers. But the estimated infection rise doesn’t seem nearly as steep as the ascension of the original Omicron variant, BA.1 (though our tracking is now poorer). XBB.1.5 also isn’t dominating equally in different parts of the country—and Truelove points out that it doesn’t yet seem tightly linked to hospitalizations in the places where it’s gained traction so far. As tempting as it may be to blame any rise in cases and hospitalizations on the latest subvariant, our own behaviors are at least as important. Drop-offs in vaccine uptake or big jumps in mitigation-free mingling can drive spikes in illness on their own. “We were expecting a wave already, this time of year,” Hill told me. Travel is up, masking is down. And just 15 percent of Americans over the age of 5 have received a bivalent shot.

The pace at which new SARS-CoV-2 variants and subvariants take over could eventually slow, but the experts I spoke with weren’t sure this would happen. Immunity across the globe remains patchy; only a subset of countries have access to updated bivalent vaccines, while some countries are still struggling to get first doses into millions of arms. And with nearly all COVID-dampening mitigations “pretty much gone” on a global scale, Hodcroft told me, it’s gotten awfully easy for the coronavirus to keep experimenting with new ways to stump our immune defenses. XBB.1.5 is both the product and the catalyst of unfettered spread—and should that continue, the virus will take advantage again.

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