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Omicron XBB.1.5 is the 'most transmissible' subvariant yet. Here's what we know

The highly infectious omicron XBB.1.5 subvariant accounts for over 70% of cases in some parts of the United States. Here's we know about symptoms and severity so far.
/ Source: TODAY

The holidays spurred a new jump in COVID-19 cases and hospitalizations, coinciding with an early, severe flu season and a wave of respiratory syncytial virus or RSV — which many are dubbing a "tripledemic" of respiratory viruses.

Just as quickly as the BQ.1 and B.Q.1.1 variants started sweeping the nation and headlines, a new COVID-19 variant began dominating in parts of the country: XBB.1.5.

XBB.1.5, which is a subvariant of omicron, now accounts for more than 27.6% of confirmed COVID-19 cases in the United States and over 70% of confirmed cases in the Northeast, according to the most recent estimates from the U.S. Centers for Disease Control and Prevention. In the week ending on Dec. 31, XBB.1.5 accounted for more than 40% of confirmed cases in the U.S. but as of the week ending Jan. 7, BQ.1.1 is back in the lead, causing 34% of cases.

“(XBB.1.5) seems to be spreading rapidly. It’s really going up and up,” Dr. Ziyad Al-Aly, a clinical epidemiologist at Washington University in St. Louis, tells

“If you go by this estimate — especially in the Northeast, but it seems to be everywhere — that velocity of change over time is already remarkably high,” says Al-Aly.

Dr. Ashish Jha, the White House COVID-19 response coordinator, expressed his concerns about the "stunning increase" in the percent of cases that XBB.1.5 is responsible for in a now-viral Jan. 4 Twitter thread.

So what is XBB.1.5, how did it become the dominant COVID-19 variant in parts of the U.S. so quickly, and how does its transmissibility and severity compare to other variants? Here's what we know so far.

What is XBB.1.5?

Omicron XBB.1.5 is a recombinant (or a hybrid) of two omicron sublineages that descended from another omicron subvariant called BA.2, according to experts at the World Health Organization. It has been detected in 29 countries so far.

It evolved from the XBB family of variants, which started driving outbreaks during the fall in Singapore and other parts of Asia, Dr. Andy Pekosz, a virologist at Johns Hopkins University, tells

"The XBB.1.5 appears to have first been detected sometime in November, although that’s still a little bit hazy as people are sequencing more viruses," Pekosz explains.

"It really started to come into play in December in terms of having this exponential growth of cases here in the U.S.," says Pekosz, adding that holiday travel and gatherings are likely contributing to the surge in cases.

While there is still a lot we don't know about XBB.1.5, the data available so far suggest that XBB.1.5 is more transmissible or infectious than other omicron variants, says Al-Aly.

In a WHO press conference on Jan. 4, the WHO technical lead on COVID-19, Dr. Maria Van Kerkhove, called XBB.1.5 "the most transmissible subvariant that has been detected yet." Kerkhove added that XBB.1.5 has "rapidly replaced other circulating variants" in the Northeastern U.S. and WHO expects "further waves of infection around the world."

How did XBB.1.5 become dominant so quickly?

In the last few months, the U.S. has seen several omicron variants — including BA.4, BA.5, BQ.1, and BQ.1.1 — co-circulating at the same time rather than one dominant strain, TODAY previously reported.

“For the longest time ... there was no clear winner, but now the writing is on the wall,” says Al-Aly, referring to XBB.1.5.

"XBB.1.5 became dominant so quickly based on its ability to escape from existing immunity and its enhanced ability to bind to host cells and infect them," Akiko Iwasaki, Ph.D., director of the Center for Infection & Immunity at the Yale School of Medicine, tells via email.

The XBB family of variants have a lot of mutations, says Pekosz, but XBB.1.5 picked up an extra mutation that allows the virus to adhere better to cells in the respiratory tract and replicate easily.

Regarding the existing immunity aspect, the body produces antibodies that detect and fight off the virus, per the CDC, which protect you from getting sick in the future. "In people who have been vaccinated or boosted, or previously infected with SARS-CoV-2, we operate under the assumption that they may carry some immunity," Al-Aly explains.

However, XBB.1.5 is better at dodging these antibodies, which is not new with the XBB family. "XBB.1.5 appears to be just as immune evasive as the XBB and XBB.1, which are more evasive of antibodies ... than other omicron subvariants," says Iwasaki.

Are the symptoms of XBB.1.5 more severe?

“So far, the symptoms are very similar to other variants, (and) increases in hospitalizations seem to be focused on the elderly, who have always been more susceptible to severe COVID," says Pekosz.

Some of the most common COVID-19 symptoms at this stage in the pandemic, according to a December report from ZOE, which has been tracking symptoms, include:

  • Sore throat
  • Runny nose
  • Blocked nose
  • Sneezing
  • Cough without phlegm (a dry cough)
  • Headache
  • Cough with phlegm (a wet cough)
  • Hoarse voice
  • Muscle aches and pain
  • Altered sense of smell

While XBB.1.5 is more transmissible than other omicron variants, initial data suggest that it is not more severe, says Al-Aly. This is good news, he adds, but not a reason to let our guard down.

"Even if it's less severe, if it's more transmissible, (then) it affects a whole lot of people — that could still fill up hospitals and also potentially increase the rates of death," says Al-Aly. This is especially true for high-risk or immunocompromised groups, the experts note.

XBB.1.5's mutations do not seem to affect the part of the virus that is targeted by the COVID-19 antiviral treatment Paxlovid. "There is no reason for us to believe that it won't respond to existing antivirals," Al-Aly adds.

That said, we do not know exactly how XBB.1.5 will compare to other omicron subvariants with respect to symptoms and severity yet, says Iwasaki. As with any new variant, we will learn more about XBB.1.5 as more data becomes available.

“The more (the coronavirus) circulates, the more opportunities it will have to change,” Van Kerkhove pointed out at the WHO press conference.

Can you get XBB.1.5 after having another omicron variant?

Yes, and one can't necessarily assume that prior infection — even in the last several months — will protect you, the experts note.

"It is distinct enough from some of the earlier omicron variants, like BA.1 and BA.5, for example, that you could theoretically get reinfected with this XBB.1.5 even if you had an earlier omicron infection," says Pekosz.

"Also over time your immune response gets reduced, and that makes it even more likely that you'll get infected," says Pekosz. So even if you had a decent immune response to omicron, by two or three months later, it will decrease and it'll be easier XBB.1.5 to be able to infect you, Pekosz adds.

Al-Aly calls the immune-evasive feature "alarming" because it may increase the risk of reinfection, which experts agree people should try to avoid.

“Repeated infections with SARS-CoV-2 appear to increase the risk for long COVID,” says Iwasaki, citing a recent study in Nature Medicine.

Can I get XBB.1.5 if I'm vaccinated and boosted?

It's possible to get XBB.1.5 even if you're vaccinated and boosted, says Al-Aly. However, the current COVID-19 vaccine and the bivalent booster (which targets BA.4 and BA.5) still provide protection against omicron variants and severe disease, the experts note.

"It might evade vaccine-induced immunity, but that doesn't mean that vaccines will all of a sudden stop protecting you. ... It just means the effectiveness is diminished," says Al-Aly, adding that being vaccinated and boosted is still absolutely safer than being unvaccinated.

"You would want to have a vaccine that’s specifically tailored to XBB.1.5, but we don’t have that (yet) — the only thing that we have right now is the bivalent booster," says Al-Aly. So it's still a good idea to be up to date on your vaccinations and boosters if you're not already.

In the meantime, you can also protect yourself and others from COVID-19 by practicing everyday precautions, like wearing a mask, avoiding crowded indoor spaces and staying home when sick, Al-Aly says.

"I think (XBB.1.5) is a reminder that this virus is going to continue to be with us, and that we're going to have to double our efforts to use the tools that we have to limit the impact of infection," says Pekosz.