The World Health Organization has its eye on a new COVID variant thought to be driving a new surge of cases in India—at a time when reported cases are down in much of the rest of the world.

XBB.1.16, dubbed “Arcturus” by variant trackers, is very similar to U.S. dominant “Kraken” XBB.1.5—the most transmissible COVID variant yet, Maria Van Kerkhove, COVID-19 technical lead for the WHO, said earlier this week at a news conference.

But additional mutations in the virus’s spike protein, which attaches to and infects human cells, has the potential to make the variant more infectious and even cause more severe disease. For this reason, and due to rising cases in the East, XBB.1.16 is considered “one to watch,” Van Kerkhove says.

It’s a warning we’ve heard before about other Omicron spawn—XBB.1.5 in particular. The variant, which rose to prominence late last year and early this year, elicited warnings that it could cause more severe disease, based on new mutations it had developed.

It was a fate that didn’t play out—though the variant certainly took the lead when it came to transmissibility. XBB.1.5 accounted for just under half of all globally sequenced cases in early March, according to the WHO.

Only time will tell when it comes to what, if any, differences in severity XB.1.16 will display. Mutations that seem concerning in theory aren’t always concerning in real life because of the highly complex nature of population immunity.

Regardless, “the rapid increase in Arcturus in India is concerning,” Ryan Gregory, a biology professor at the University of Guelph in Ontario, Canada, tells Fortune. He spearheaded the development of “street names” for COVID variants as it became clear that the WHO wouldn’t be assigning new Greek letters for them.

That XBB.1.16 is apparently gaining steam in a country with hefty population immunity from both prior infection and immunity is concerning, Gregory notes. While it’s not clear how big of a surge the new variant may cause in India or elsewhere, “large waves aren't the main pattern” of COVID cases anymore, he says. “It's the consistently high baseline that won't come down."

Here’s what we know so far about the latest variant to raise eyebrows—and the first to do so in several months.

When and where was XBB.1.16 discovered?

XBB.1.16 was added to the WHO’s list of variants under monitoring just recently, on March 22. COVID surveillance is at an all-time low. But so far, the bulk of cases have been identified where the new variant was first spotted, in India—one of the few countries where recorded COVID cases are on the rise, according to the WHO.

Where else has XBB.1.16 been seen?

From reported sequences, we know that the variant has also been spotted in the U.S.—in California, New Jersey, Virginia, Texas, Washington, New York, Illinois, Minnesota, Georgia, Florida, Pennsylvania, Ohio, Nevada, Indiana, North Carolina, Louisiana, and Delaware, to be precise.

A descendant variant, XBB.1.16.1, has also been seen in Nebraska, Missouri, and Michigan.

Neither new variant accounted for enough sequences (1% nationally) to be listed in the U.S. Centers for Disease Control and Prevention’s updated COVID forecast on Friday. Sequences of both are still aggregated under XBB, which is estimated to account for 2.5% of current cases in the country.

XBB.1.16 and its descendents have also been identified in Singapore, Australia, the United Kingdom, Japan, Israel, Canada, Malaysia, Denmark, New Zealand, Germany, South Korea, Spain, the Netherlands, Thailand, Sweden, South Africa, Italy, and China.

How did XBB.1.16 evolve?

The variant is a recombinant, or combination, of two descendants of so-called “stealth Omicron” BA.2. When compared to “grandparent” lineage XBB, it has three additional mutations, according to the WHO.

And it's picking up additional mutations, Raj Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., and a top COVID variant tracker, tells Fortune.

Is XBB.1.16 causing hospitalizations and deaths to rise?

So far, no lab studies regarding the variant’s disease severity have been completed. Reassuringly, hospitalizations, ICU admissions, and deaths have not yet risen due to the variant, according to a Thursday COVID-19 situational report by the WHO.

While the variant isn’t causing “large clusters” of illness, it is fueling a “steady increase in fresh cases” throughout India, Ma Subramanian, health minister for Tamil Nadu, an Indian state, said Friday, The Times of India reported.

So why is XBB.1.16 so concerning?

If the new variant is indeed capable of fueling a rise in hospitalizations or deaths, it may be too early to tell. Upward trends in such metrics can take weeks to materialize, which is why they were known early in the pandemic as “lagging indicators.”

XBB.1.16 has two new mutations in particular that “makes it fitter than any variant so far,” Rajnarayanan says. Those two mutations don’t exist on relative XBB.1.5.

“Currently, XBB.1.16 is the big dog,” he says. “It’s picking up mutations that are common in other variants that will increase its advantage further.”

Particularly concerning is mutation K478R, which may make the variant better at overcoming antibodies from prior infection and vaccination, making people sicker, and spreading in general, according to the WHO.

Additionally, XBB.1.16 has shown an ability to quickly outpace U.S.-dominant XBB.1.5 when it comes to spread. The new variant has shown a 188% growth advantage over it in past three months, Rajnarayanan says.

And in India, where XBB caused the last big wave, XBB.1.16 is displaying a 64% growth advantage, he adds.

Will the new Omicron COVID booster protect me from XBB.1.16?

Such boosters should offer “some protection if the dose is recent,” Rajnarayanan says, though the virus has continued to evolve since they were released last fall.

Paxlovid continues to work, he adds. But when it comes to monoclonal antibodies, “the rest of the toolkit is useless,” he says.

Why should I care about XBB.1.16—or any COVID variant—anymore?

It’s a fair question, Rajnarayanan says: “With so much going on, everyone is kind of tired and has given up.”

Unfortunately, it’s not safe to assume that therapeutics like COVID antiviral Paxlovid, vaccines, and tests will continue to work as viral evolution continues, he cautions.

“People want to know, will vaccines work? Are there tools? Should I mask?” he says. The answers are subject to change, however, and depend on what variants are circulating, and at what volume.

It’s important to stay vigilant, he says.

This story was originally featured on Fortune.com

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