XBB is getting a lot of buzz because it spreads fast—and seems to be able to evade immunity that people have built up from having a previous COVID-19 infection or getting the vaccine, says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. Still, Dr. Schaffner says, "it's early days and we have a lot to learn."

Here's what we know about XBB so far, and why doctors are keeping a close eye on it.

What is the XBB COVID variant?

XBB is one of the "new class" of Omicron variants that are spreading fast right now, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. That includes BQ.1.1, BQ.1, BQ.1.3, BA.2.3.20, and XBB, he says.

"XBB is a hybrid version of two strains of the BA.2 form of Omicron," explains Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. It's currently "spreading efficiently in Singapore," he adds.

The variant was first detected in August 2022 in India, and has been detected in more than 17 countries since then, including Australia, Bangladesh, Denmark, India, Japan and the U.S., per Singapore's Ministry of Health.

XBB is thought to have the best ability to evade antibody protections of these newly emerged COVID variants, according to a pre-print study from researchers in China. That study said that the new strains of Omicron, and XBB in particular, "are the most antibody-evasive strain tested, far exceeding BA.5 and approaching SARS-CoV-1 level." (SARS-CoV-1, in case you're not familiar with it, is the strain of coronavirus that causes SARS, a respiratory virus that can cause severe illness.)

Meaning, the vaccine and having previously had COVID-19 are not thought to offer the same level of protection against XBB as they have with previous strains of COVID-19. Antibody drugs like Evusheld and bebtelovimab may also not be very effective against XBB, the pre-print study says.

"These variants are evolving to evade protection," Dr. Russo says. The bivalent booster is "likely going to be protective against severe disease" with XBB, but will be "imperfect against preventing infection," Dr. Russo says.

Don't panic, though. "When it comes to evasion of vaccine protection, it's important to recognize that vaccine protection is not all or none," Dr. Adalja says. "Even with immune-evasive variants, vaccine protection against what matters most—severe disease—remains intact."

XBB variant symptoms

So far, symptoms of XBB seem to be similar to what they've been with COVID-19 in general. According to the Centers for Disease Control and Prevention (CDC), those can include:

· Fever or chills

· Cough

· Shortness of breath or difficulty breathing

· Fatigue

· Muscle or body aches

· Headache

· New loss of taste or smell

· Sore throat

· Congestion or runny nose

· Nausea or vomiting

· Diarrhea

How contagious is the XBB subvariant?

Like other strains of Omicron, XBB is thought to be very contagious. Singapore's Ministry of Health notes that the variant now makes up 54% of COVID-19 cases in the country, up from 22% the week before.

Singapore's Ministry of health says that XBB is "at least as transmissible as currently circulating variants" but adds that "there is no evidence that XBB causes more severe illness."

When will the XBB subvariant peak?

There are a lot of unknowns about XBB right now. While it's been detected in the U.S., BA.5 and BA.4.6 continue to be the dominant variants in this country, per CDC data.

Other variants are also started to spread at the same time, Dr. Adalja says, and it's unclear which will displace BA.4.6 and BA.5 in the U.S., if they will at all. "It's likely to spread to some degree in the U.S. but unclear if it—or some other related variant such as BQ.1.1—will become dominant," he says.

Dr. Schaffner says there is "some concern" about XBB and fellow variants on the rise. "Watching what happens over the next several weeks is important," he says.