XBB.1.5: What to Know About the 'Most Transmissible' COVID Subvariant Yet

Throughout the pandemic, the virus that causes COVID-19 has continued to mutate and change as it spreads to become more transmissible. The newest subvariant picking up steam in the U.S.—called XBB.1.5—is no exception.

XBB.1.5 is yet another Omicron subvariant, similar to BA.5, BQ.1, and other strains that have dominated in recent months. But XBB.1.5 contains some mutations that allow it to spread more efficiently, outpacing its other Omicron competitors, and prompting some to call it the “Kraken” variant.

For the week ending January 7, XBB.1.5 was estimated to have made up over a quarter of COVID infections across the country. It’s even more dominant in the Northeast, where it makes up over 70% of cases.

Experts are still learning more about XBB.1.5 and how it might impact the country in the upcoming weeks, but even with its quick rise in dominance over the last 6 weeks, the new subvariant shouldn’t be a cause for panic, explained Shira Doron, MD, chief infection control officer for the Tufts Medicine Health System and epidemiologist and infectious disease physician at Tufts Medical Center.

“Since the big massive Omicron wave of a year ago, we have seen every couple of months, a new subvariant takes over, becomes predominant,” Dr. Doron told Health. “Often there’s a bump in cases and hospitalizations concomitant with that change over to the next variant [or] subvariant, but it’s not particularly concerning or alarming.”

However, experts have been watching the new subvariant to see to what extent the virus can both evade immunity and infect cells more easily, which could drive cases up—even among people who have already had COVID or who are vaccinated.

Here’s what experts know about XBB.1.5 so far, how it fares in the face of common prevention and treatment measures such as vaccines and Paxlovid, and what people can consider doing to stay safe this winter. 

A Much More Transmissible Version of COVID

Because it’s descended from other Omicron subvariants, in many ways XBB.1.5 is not all that different from what the U.S. has been experiencing for the last 13 months, said William Schaffner, MD, professor of infectious disease at Vanderbilt University.

“XBB.1.5 we can think of as an offshoot of Omicron—it’s part of the greater Omicron family,” Dr. Schaffner told Health. “You can almost think of it as a grandchild of Omicron.”

More specifically, XBB.1.5 is a descendant of the BA.2 variant that was dominant in the spring of 2022. From the BA.2 variant came the XBB subvariant that was first identified last fall, said Peter Chin-Hong, MD, professor of medicine and infectious disease specialist at the University of California, San Francisco.

“This XBB evolved to XBB.1.5, first identified in New York in October of 2022. But then, [it] probably originated somewhere in the Northeast in the United States. So it’s our own homegrown variant,” he told Health. “Now it seems to be taking over much of the Northeast, spreading across the country, and [in] some parts of Europe as well. And [it’s] thought to really be a threat, trying to seek global dominance.”

The reason for this, Dr. Chin-Hong explained, is because XBB.1.5 is even more adept than XBB at evading immunity and getting into cells. It could be a possible explanation for why COVID cases are starting to creep back up after an October and November lull.

“In terms of immune slipperiness—which was why XBB was initially feared—[XBB.1.5] adds one additional superpower, which is having an easier time to infect cells,” Dr. Chin-Hong said. “That’s because of that mutation that you’ve probably heard about—F486P. It just makes it bite on to cells through the ACE-2 receptor more efficiently than XBB. So it’s like XBB on steroids.”

These mutations can make XBB.1.5 sound worrying, but it’s important to put the new subvariant in a larger context, Dr. Doron added. Any time a new variant outpaces another, it’s doing so because it’s spreading more efficiently and infecting more people, which is the virus’ “goal,” so to speak.

“Any variant that’s gaining a growth advantage is probably doing so due to immune invasiveness, inherent increased transmissibility, or both,” she said. “That is what’s going to happen forever and ever and ever, because that’s what this virus does—it mutates to give itself a growth advantage.”

By All Other Accounts, XBB.1.5 is Nothing New

The XBB.1.5 subvariant is “the most transmissible subvariant that has been detected yet,” Maria Van Kerkhove, PhD, technical lead on COVID-19 at the World Health Organization, said in a media briefing on January 4. However, in many other ways it is quite similar to the Omicron variants and subvariants that the U.S. is used to.

“It’s of the Omicron lineage, so it generally acts like Omicron as much as we know it. So in terms of symptoms—the usual sore throat, nasal stuffiness, headache, flu [or] cold-like symptoms for many people, some fevers in some individuals,” Dr. Chin-Hong said.

Though symptoms are generally mild, Dr. Schaffner added, that doesn’t always mean that they’re no big deal.

“[Mild is] defined as less serious than hospitalization, but it can put you in the bed for a couple of days,” he said. “Even though you’ve been infected or even though you’ve been vaccinated, you can still acquire this mild infection.”

The other fortunate thing about XBB.1.5 being so similar to other Omicron variants is, along with more predictable symptoms, tried and true prevention and treatment methods still, for the most part, are effective against the new subvariant.

Monoclonal antibody treatments, however—used earlier on in the pandemic to help people who may have not had an adequate immune response to the virus—are falling by the wayside as these newer Omicron variants get better at evading the immune system, Dr. Doron said.

But besides this, all other common prevention strategies and treatments seem to work against XBB.1.5, experts agreed.

“The vaccines are still the best thing that we have for preventing people from getting severe disease from COVID and that includes XBB.1.5,” Adam Ratner, MD, director of pediatric infectious diseases at Hassenfeld Children’s Hospital and NYU Langone Health, told Health.

The fact that hospitalizations from COVID have not spiked in recent weeks is a good indicator that the vaccines can still protect against severe disease, even with this new variant circulating, Dr. Ratner added.

Other preventions, such as social distancing and mask use, just stop a person from passing the virus to other people, so they’re still useful tools no matter what kind of respiratory virus or variant a person has. And drug treatments such as Paxlovid and remdesivir work regardless of the specific spike protein on a variant, so they will continue to work against XBB.1.5 as well, Dr. Chin-Hong said.

Living With The Virus This Winter 

The reason that the U.S. likely won’t see overwhelmed hospital systems or other issues thanks to XBB.1.5 has a lot to do with the high levels of immunity that already exist in the population at large, the experts explained.

“We’re in a different place than we were a couple of years ago because many, many more people have had experience with COVID. And I mean that either as they’ve had COVID, or they’ve been vaccinated against COVID,” Dr. Ratner said. “I don’t think we’re going to see the same kind of surge of hospitalizations that we’ve seen in the past.”

It’s possible that overall hospitalization rates will go up as cases increase, Dr. Doron said, though that’s to be expected.

Raw hospitalization data, depending on where it comes from, may also slightly overcount how many people are truly undergoing serious COVID infections. According to Dr. Doron, people can seek emergency care for something unrelated to COVID, but still test positive for the virus during their stay—essentially, they’re being treated at the hospital while they have COVID, as opposed to being treated for it, which can skew metrics.

Though hospitalization rates may not be the perfect metric for determining how well the U.S. is handling a new surge in COVID cases, it’s certainly somewhere to start. The Centers for Disease Control and Prevention (CDC) has a map that shows which counties are high, medium, or low risk—people can use this as a general guidepost for when they might want to increase preventative measures such as masking or social distancing, or dial them back.

“Given that there’s a highly transmissible variant spreading now, I think it’s reasonable to think about risks, especially if you or someone in your family is at higher risk of severe disease—so someone who’s elderly, someone who’s immunocompromised, something like that,” Dr. Ratner said.

It’s really that last group who experts are most focused on right now—seniors and those who are at an increased risk of developing severe complications from XBB.1.5 should get the bivalent booster as soon as possible, Drs. Chin-Hong and Schaffner said.

“Such a large proportion of our population—including the high risk population, that is high risk for serious disease—has not yet availed themselves of the bivalent new updated booster. That’s really unfortunate,” said Dr. Schaffner. “For example, there’s a new [preprint] study that’s come out of Israel just released recently, and it shows that if [seniors] do take the bivalent vaccine, [their] risk of hospitalization is reduced by over 80%. I mean, that’s really still a very, very strong vote of confidence in the vaccine.”

And of course, with how transmissible XBB.1.5 is, the subvariant could have larger impacts around the globe, too. China, which has seen a sharp increase in cases over the past several weeks, could be greatly affected by XBB.1.5. Dr. Chin-Hong said, which could impact the global economy and safety.

XBB.1.5 may also make it incredibly difficult for people to avoid getting infected with COVID, he added. But a commitment to vaccines, masking in high risk situations, and keeping up with our general health should ensure that hospitalization rates remain manageable and people recover from a mild XBB.1.5 illness if or when they do catch it.

“The virus will continue to produce variants,” Dr. Schaffner said. “We hope that they—like XBB.1.5—are transmissible but not more severe.”