A new COVID-19 variant, XBB.1.16 or "Arcturus," has emerged, and the  World Health Organization  (WHO) has labeled it as a variant "to watch" as it drives a new increase in cases in India and spreads to more countries worldwide, including the United States.

What is the XBB.1.16 variant?

XBB.1.16 is a recombinant variant, meaning it evolved from two other variants, likely when a single person was infected by two or more coronavirus variants at the same time. XBB.1.16 evolved from BA.2.10.1 and BA.2.75.

Compared to the parent XBB lineage, XBB.1.16 has three additional mutations on its spike protein. Although this newest variant is similar to XBB.1.5, which currently makes up around 85% of U.S. cases and 45% of global cases, XBB.1.16 has an additional mutation that has been associated with increased infectivity and a potential increase in pathogenicity.

"Currently, XBB.1.16 is the big dog," said Raj Rajnarayanan, assistant dean of research and an associate professor at the  New York Institute of Technology  campus in Arkansas. "It's picking up mutations that are common in other variants that will increase its advantage further."

"This is one to watch," said Maria Van Kerkhove, WHO's technical lead for COVID-19 response. "We're monitoring it because it has potential changes that we need to keep a good eye out on."

According to WHO, XBB.1.16 has been circulating worldwide for a few months, and the organization labeled it as a variant "under monitoring" on March 22. So far, roughly 800 cases have been sequenced in over 20 countries, including India, Singapore, Australia, the United Kingdom, Canada, and the United States.

Most XBB.1.16 cases have occurred in India. Ma Subramanian, health minister of the Indian state of Tamil Nadu, said that while the variant hasn't caused "large clusters" of illness, it is still driving a "steady increase of fresh cases."

"[T]he rapid increase in Arcturus [XBB.1.16] in India is concerning," said Ryan Gregory, a biology professor at the  University of Guleph in Canada, noting that the country has significant population immunity against COVID-19 from both prior infection and vaccination.

Although current reports do not show an increase in hospitalizations, ICU admissions, or deaths from XBB.1.16, Van Kerkhove said that "we have to remain vigilant" as the variant continues to spread.

How will this variant impact America?

In the United States, cases of XBB.1.16 have been reported in 18 states, including California, New Jersey, New York, Texas, and Virginia. A descendant of this variant, XBB.1.1.16.1, has also been reported in several states, including Michigan, Missouri, and Nebraska.

Currently, XBB.1.16 and XBB.1.16.1 are still aggregated under XBB on  CDC's  COVID-19 variant  tracker, making up around 2.5% of all COVID-19 cases in the country as of April 1.

Although XBB.1.16 is relatively uncommon in the United States right now, Rajnarayanan said that it has the potential to quickly outpace the currently dominant XBB.1.5 variant. Over the last three months, XBB.1.16 has shown a 188% growth advantage, and in India, where it is currently driving a new increase in cases, the variant has shown a 64% growth advantage.

So far, it's not clear whether XBB.1.16 will lead to a new wave in the United States, and it may take weeks for an increase in cases to be apparent, if it does occur.

According to Rajnarayanan, omicron-targeted boosters should offer "some protection if the dose is recent," and Paxlovid could continue to work as a treatment. However, other COVID-19 treatments, such as monoclonal antibodies, are unlikely to have an effect on XBB.1.16.

Although "everyone is kind of tired and has given up" on keeping up with new coronavirus variants, Rajnarayanan warns that it's important to be vigilant since it's possible that COVID-19 tests, vaccines, and antivirals could become ineffective as the virus continues to evolve.

"People want to know, will vaccines work? Are there tools? Should I mask?" Rajnarayanan said, noting that the answers to these questions will continue to change as new variants evolve and circulate. (Hein, MedPage Today, 4/4; Hollowell, Becker's Hospital Review, 4/3; Prater, Fortune, 3/31; Carbajal, Becker's Hospital Review, 4/3; Schnirring, CIDRAP News, 3/31)