What we know about XBB.1.5, the new dominant Omicron subvariant

A new version of the Omicron variant of the coronavirus is quickly gaining ground in the U.S. The subvariant, called XBB.1.5, has nearly doubled over the last week, claiming a lead over the BQ.1 and BQ.1.1 subvariants which had just gained dominance in November.

BQ.1 and BQ.1.1 led to a gradual rise in cases and hospitalizations but did not cause a significant wave of infections like the Omicron BA.5 subvariant did last summer. But XBB.1.5, some experts say, could potentially lead to a bigger surge in cases because of how quickly it’s spreading.

“Currently XBB.1.5 has a 120% weekly growth advantage, which equates to, on average, 1 infected person infecting 2 others,” Dr. Katelyn Jetelina, an epidemiologist and author of the popular Substack column “Your Local Epidemiologist,” recently wrote. “This rate is higher than we’ve seen with any other subvariant this year given our immunity wall,” she added.

According to the latest data from the Centers for Disease Control and Prevention, XBB.1.5 is now responsible for 40.5% of new cases nationwide. In the Northeast, the agency estimates the subvariant is causing 75% of new cases.

It isn’t clear yet where this version of the virus originated from, but according to the World Health Organization, it has been identified in more than 25 countries so far, and its presence is increasing here in the U.S. and in Europe.

Eric Topol, a physician-scientist and director of the Scripps Research Translational Institute, wrote in a post on his Ground Truths Substack that in the United States, “New York is the bellwether for what is happening with XBB.1.5 and it doesn’t look good.” He noted that hospitalizations, especially among seniors, have been on the rise since the new subvariant took hold in that state. However, he said there are also many other factors that may be contributing to that increase, including waning immunity, cold weather (more people spending more time indoors), holiday gatherings and lack of mitigation measures.

Other experts have noted that hospitalizations have risen overall across the country, not only in areas where XBB.1.5 is present, so it’s difficult to say with certainty that the new Omicron strain is what’s driving the increase in COVID-related hospitalizations right now. Nevertheless, experts expect XBB.1.5 to continue spreading. How big or severe a wave it will cause is not clear yet, but some models indicate cases driven by the new subvariant could peak in February.

Is XBB.1.5 more transmissible? Can it cause more severe disease?

There is a lot that is still unknown about XBB.1.5, but experts have said it does look like it is more transmissible than other Omicron subvariants. “The ones that start becoming predominant usually are,” Dr. Monica Gandhi, an infectious disease specialist and a professor of medicine at the University of California, San Francisco, told Yahoo News.

Maria Van Kerkhove, the WHO’s COVID-19 technical lead, told reporters during a press conference in Geneva on Wednesday that XBB.1.5 “is the most transmissible subvariant that has been detected yet” and that her organization was conducting an assessment to determine its risks. That information, she said, should be available in the coming days.

One characteristic of XBB.1.5 that may give it an advantage to spread more easily is a mutation in the spike protein that lets the virus bind more tightly to the human angiotensin-converting enzyme (ACE2) receptor — the door the virus uses to enter our cells. Kerkhove said this allows for the virus to adhere to cells and replicate more easily.

“Anything that lets you enter the whole cell more quickly is more transmissible basically, [and] binds more tightly ... and then you know, you get infected,” Gandhi said.

So far, there is no evidence suggesting that XBB.1.5 can make people sicker than previous versions of the virus. Health experts also hope that even if XBB.1.5 causes a wave of infections, its impact won’t be as damaging, given that our population now has a higher level of immunity either from vaccination, infection or both. This immunity, Gandhi notes, may not protect some people from getting infected, but it should continue to protect against the severe outcomes of the disease.

How are our vaccines holding up against XBB.1.5?

There’s no data available yet on how well the COVID-19 vaccines neutralize XBB.1.5, but White House coronavirus response coordinator Dr. Ashish Jha tweeted that this information will soon be available.

What we do know is that XBB.1.5 is the product of the recombination of two descendants of the Omicron BA.2 subvariant, which drove a small wave of cases in the U.S. last spring, and a new sublineage called XBB.

Studies have found that XBB is one of the most immune-evasive strains of the coronavirus yet. Since XBB.1.5 is its close relative, experts predict it will also be capable of evading antibodies from vaccines, COVID infections or both.

But Gandhi explained that our vaccines should remain effective at preventing severe illness and death because aside from antibodies, there are other parts of the immune system that are activated with vaccination and infection, such as B cells and T cells, which can also protect us against the virus.

“B cells, you know, when they see a new variant or a new subvariant ... they turn out antibodies that are adapted to whatever they see in front of them,” Gandhi said. “So if they see a mutated spike protein of XBB.1.5, I mean it’ll take them a couple days to make those antibodies, but when they make those antibodies, they’re directed against the virus that they see,” she added.

What can people do to protect themselves from XBB.1.5?

Gandhi and other experts have said the best way to stay protected against these emerging variants is to stay up to date with COVID-19 vaccinations, which includes getting the new bivalent booster.

In a recent study published in the New England Journal of Medicine, the bivalent boosters developed by Pfizer and Moderna were found to bolster antibody responses to many subvariants of Omicron, including the XBB variant. Given that XBB.1.5 is a relative of XBB, it is also likely that these Omicron boosters offer protection against the new subvariant.

To stay protected against severe disease this winter, U.S. health officials have encouraged everyone to get the bivalent booster who is eligible for it. Gandhi said that for people aged 65 and older, who are the most vulnerable to the virus, getting boosted is particularly important.

“For those 65 and up at least, I would really recommend a booster anytime a new variant comes along because there’s just gonna be more circulating virus,” she said.

The majority of COVID-19 hospitalizations in recent weeks have been among older adults, particularly those who have not received a booster shot in the past six months. Yet, only 38% of those in that age group have received the updated Omicron booster, according to the latest CDC data.

At this time, masks are no longer required in many areas of the country, but the CDC continues to recommend masking for residents of counties that have high community levels of COVID-19, as well as for anyone who may be immunocompromised or at increased risk of severe disease.

At a press briefing last month, Dr. Rochelle Walensky, director of the CDC, said people traveling by plane or taking any form of public transportation should also consider masking.

Gandhi said that variants will continue to emerge because the coronavirus will not be completely eliminated, but there’s no reason to panic, because we now have tools to fight the virus.

“I think that people should realize that this is not an eradicable infection. Unfortunately, we are always going to have variants ... I think we will always have to keep up, you know, with our vaccines, especially those who are older [or] the most at risk, [with] both boosters and therapeutics, but we’re gonna live with this virus,” Gandhi said.