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New COVID Variant "Spreading Like Wildfire" in USA: "Most Transmissible One Yet"

Here’s what we know about XBB.1.5.
FACT CHECKED BY Emilia Paluszek

New COVID-19 Omicron variant XBB.1.5 is being called the most transmissible to date, with new infections caused by the variant rising from 4% to 41%, just over the month of December. "That's a stunning increase," says White House COVID-19 Response coordinator Dr. Ashish Jha. "If you had an infection before July OR your last vaccine was before bivalent update in September, our protection against an XBB.1.5 infection is probably not that great. We will soon have more data on how well vaccines neutralize XBB.1.5. But right now, for folks without a very recent infection or a bivalent vaccine, you likely have very little protection against infection. And for older folks, diminishing protection against serious illness."

According to the CDC, XBB.1.5 made up 75.3% of COVID-19 cases in northeastern states during the last week of December, and shows no signs of slowing down. "It's definitely the majority of cases in the Northeast, and we expect that to be for the whole country," says Dr. Shad Marvasti, associate professor and director of public health and prevention at the University of Arizona College of Medicine – Phoenix. "In Arizona, my expectation is that it's going to climb pretty quickly, especially since we have a lot of winter visitors here this time of year, and we're going to have more folks with the Phoenix Open and the Super Bowl. Viruses always want to be more and more transmissible and infect more hosts."

So how can people protect themselves from the new variant? "Make sure you are up to date on the bivalent booster (critical)," says Dr. Jha. "Test before large gatherings or before seeing someone vulnerable (I did over the holidays). Wear a high-quality mask in crowded indoor spaces. Work to improve ventilation/filtration in indoor spaces. If you have symptoms: test right away and avoid hanging out with high risk folks. If you do get Covid – immediately get evaluated for treatments. They are lifesavers."


Why Is XBB.1.5 So Transmissible?

Woman outdoor with cold and handkerchief

According to experts, XBB.1.5 has a very strong affinity to ACE2, a transmembrane protein and receptor which allows the virus to bind more easily while evading immunity, therefore boosting transmissibility. "It is the most transmissible subvariant that has been detected yet," says Maria Van Kerkhove, the WHO's Covid-19 technical lead. "The reason for this are the mutations that are within this subvariant of Omicron allowing this virus to adhere to the cell and replicate easily."

XBB.1.5 has been detected in 29 countries so far, but Van Kerkhove believes that's just the minimum. "The more this virus circulates the more opportunities it will have to change," she says. "We do expect further waves of infection around the world but that doesn't have to translate into further waves of death because our countermeasures continue to work."


Is XBB.1.5 More Virulent?

Young woman sitting alone on her sofa at home and coughing.

Thankfully, while XBB.1.5 is proving more transmissible than past variants, there is no evidence so far that it is more virulent or deadly. "The vaccines are holding. That's good," says Dr. Jeremy Luban, an expert on viruses at the University of Massachusetts Chan Medical School. "[But] anytime you have a new creature that spreads like this, person to person so rapidly, it puts a big stress on our infrastructure – medical facilities and personnel who are chronically short-staffed. That's a problem for everybody." Experts say that those up to date on their vaccines shouldn't panic. "I think there's less to be worried about," 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."

 "They are seeing some increase in hospitalizations in older folks in the Northeast, but that seems to be at least based on what you know, I'm reading and hearing from folks, that seems to be based as much on the fact that they're seeing just a surge overall… and so with that increase, it seems like we're just seeing more older people get infected and if they're not up-to-date, especially with their vaccines, they are somewhat more likely to be hospitalized," says  said Chicago Department of Public Health Commissioner Dr. Allison Arwady. "But no, I haven't seen anything suggesting that this new subvariant is clearly making people sicker. I think we'll learn more as we follow it."


Symptoms of XBB.1.5

Sick woman holding her throat.

Symptoms of XBB.1.5 haven't changed significantly from previous variants. "COVID is showing up very much like it already has. I think, if anything, we are seeing it a little bit less likely to have the more severe symptoms," says Dr. Arwady. "Definitely people get the severe symptoms still, especially if they're not up-to-date with their vaccines. But more often now we are seeing people – especially if they're fully up-to-date, maybe it's their second time getting COVID, whatever it might be – we're seeing more people actually just have cold-like symptoms, less likely to have those flu-like, really feeling very sick, the high fevers."

So far, there are no signs XBB.1.5 is leading to severe infection but experts are warning that this could change. "It does remind me of a year ago when Omicron came in over a two-to-four week period and displaced everything," Boston Medical Center's Dr. David Hamer says. "This is sort of going that direction, but I think we need more data. Over the next two-to-four weeks we'll see what happens."


Boosters Still Work

Woman in medical protective mask getting injection in arm vaccination.

As the new subvariant is still within the Omicron family, the bivalent booster is still important and effective, experts say. "It does look like the vaccine, the bivalent booster is providing continued protection against hospitalization with these variants," says Andrew Pekosz, a virologist at Johns Hopkins University. "It really emphasizes the need to get a booster particularly into vulnerable populations to provide continued protection from severe disease with these new variants."

"We aren't in 2020, but people still do need to take this seriously and protect themselves," says Dr. Barbara Mahon, director of CDC's Coronavirus and Other Respiratory Viruses Division. "With the monovalent boosters your neutralizing antibodies are just not that potent against the variants, but the bivalent booster ensures it's slightly better," says Mehul Suthar, an associate professor at the Emory Vaccine Center. "It's not knock your socks off amazing, but it's better, which says the bivalent boosters are working the way they should."


What About Treatment Options?

Nurse checking a vial of medicine.

Dr. Arwady warns that recent strains might make treatment difficult if not impossible. "As this virus continues to mutate, what we have seen is that all of what are called our monoclonal antibodies, our clinical monoclonal antibodies are inactive against these most recent subvariants," Dr. Arwady says. This is particularly concerning for the immunocompromised.

"And the monoclonal antibodies are these IV infusions that people with really severe immunocompromised will often take people who have a organ transplant, for example, and the vaccine doesn't work very well because their whole immune system has been tamped down and even though they take the vaccine, it's safe, they don't mount enough of an immune response, and so we've historically used what's called a monoclonal antibody, which helps give them passive protection, but the problem is that as these new subvariants have gotten further and further from the original…the virus has figured out how to mutate around them. So we have lost most of that monoclonal antibody treatment."

Ferozan Mast
Ferozan Mast is a science, health and wellness writer with a passion for making science and research-backed information accessible to a general audience. Read more about Ferozan