Since Omicron became the world’s dominant Covid variant, it’s taken on a lot of shapes. First there was BA.1, then BA.5, and eventually others, including BQ.1 and BQ.1.1. Now all eyes are on another scrambled string of letters and numbers—XBB.1.5, also known as the Kraken, which has swept the northeastern US in recent weeks.

The World Health Organization (WHO) has deemed XBB.1.5 the most transmissible version of the Omicron variant to date and announced that countries should consider recommending masks for risky situations, like flights. It’s quickly becoming dominant in parts of the US, and some experts are worried it may be able to dodge immunity from past infections and possibly vaccines.

Any time a new variant snowballs so quickly, it garners attention. Significant variations of the SARS-CoV-2 virus can mean more illness, hospitalizations, and death, which can strain health care systems and increase rates of long Covid. While XBB.1.5 infections are swelling, the WHO says there’s no evidence that this variant’s mutations would result in more severe infections—but it’s still early. In the US, Covid hospitalizations are ticking upward but are nowhere near their early 2022 peaks. Still, the rise of a fast-moving variant puts attention back on an ongoing problem: how vaccines should be updated. 

“For a while now, we haven’t seen a sublineage that’s taken off at that speed, so that’s another sign that this one might be worth watching for,” says Pavitra Roychoudhury, director of Covid-19 sequencing at the University of Washington Virology Lab. Roychoudhury says it’s important to get eyes on variants early to identify them and consider how to design future vaccines: “Until the time when we have a vaccine that will be effective against all variants, we’ll have to try and design them based on what is likely to be circulating at high frequency.” 

This variant is a sublineage of a recombinant of two other Omicron offshoots. That mixing can happen when a person is simultaneously infected with two variants of the virus or if the two meet in wastewater. 

This one could stand out among the several circulating Omicron variants if it turns out to have two advantages that would make it highly infectious—an ability to evade antibodies acquired from past infections or vaccinations, and strength in binding to ACE2 receptors, where Covid enters cells and infects people. A preprint posted in early January by Chinese researchers focusing on XBB.1.5 argues that it does, but that paper has not yet been published or peer reviewed. 

“It’s kind of a one-two punch of mutations,” says Peter Hotez, codirector of the Texas Children’s Hospital Center for Vaccine Development and dean of the National School of Tropical Medicine at Baylor College of Medicine. “It not only has the immune escape properties, but it was able to do it while preserving its ability to bind to the receptor.” 

It’s also spreading faster because of how people are behaving: Few are wearing masks compared to 2020, and many have traveled and gathered indoors to celebrate the holiday season. That’s a recipe for lots of people getting sick, fast. “What we’re having right now is this subvariant that has a lot of immune escape that is also coming into play when we have pretty much removed most, if not all, of our other public health mitigating practices,” says Stephanie Silvera, an epidemiologist and professor of public health at Montclair State University in New Jersey. 

So far, the regional differences in XBB.1.5 are stark. According to the US Centers for Disease Control and Prevention, the variant was projected to make up more than 70 percent of the cases found in New England and New York state but fewer than 5 percent of the cases in the Midwest and West Coast of the US and about 27 percent of national cases for the week ending January 6. Other lineages of the virus are expected to have slow or no growth in the proportion of cases they make up. 

The variant has been found in 38 countries, according to the WHO. Those include South Korea, Australia, and nations in Europe, where it made up less than 2.5 percent of the proportion of cases during the final two weeks of 2022, the European Center for Disease Control and Prevention said Monday. But the agency notes that the rapid spread in the US doesn’t mean the variant will also become dominant across the Atlantic. 

While most countries have seen Covid waves rise and fall, China is currently dealing with its first massive outbreak after three years of strict containment measures. A hyper-contagious variant could affect that population differently, and experts don’t know yet if unfettered spread of XBB.1.5 in China could spur the emergence of a “new supervariant” or one that’s especially dangerous or transmissible.  

News of a novel variant doesn’t necessarily mean more danger—even when they’re nicknamed after a sea monster. The Kraken name comes from a biology professor who has taken on the job of demystifying Covid variants by giving them mythological nicknames instead of hard-to-follow numerals. Throughout the pandemic, researchers have fought back against giving too much attention and fearful names to “scariants,” which could make people panic before scientists know more about the virus’s new forms—as well as against nomenclature that is overly complex, racist, or xenophobic.

And some variants never live up to initial concerns. In late summer of 2021, experts watched the Mu variant closely, fearing it would cause breakthrough infections. But it receded quickly. XBB.1.5 could fizzle out without becoming dominant elsewhere, but experts say it’s too soon to know how it will spread. 

XBB.1.5 is also another reminder that viruses can mutate in unexpected ways—and do so far more quickly than scientists can tweak vaccines in response. “We almost never see a perfect match between a vaccine and a respiratory virus that’s circulating,” says Andy Pekosz, a virologist at Johns Hopkins University. When selecting a vaccine, health officials, like the US Food and Drug Administration, must make decisions about what sequence they will attempt to target. “That’s not going to be an easy decision,” Pekosz says. 

An alternative approach is to develop a vaccine that works against all coronaviruses, or specifically against members of the SARS lineage, which would be less vulnerable to outdating when new variants arise.

Vaccines are still vital tools to protect people from severe illness, particularly the most recent bivalent Covid booster that was released last fall in the US, Europe, and other countries to better fight Omicron infections. But just 15.4 percent of people over the age of 5 in the US have had an updated Covid booster shot, according to the CDC. And that makes it hard to see the true effect of the variant on the vaccinated. 

Even with the future of XBB.1.5 and Omicron unknown, experts say the tools for fighting Covid haven’t changed. Getting vaccinated, masking, and taking antivirals like Paxlovid if infected can all help. “The message is what the message has been: We need to have a healthy level of concern,” says Silvera. “Somewhere between pretending the pandemic is over and outright panic is where we should all be landing.”