What to Know About the New XBB.1.16 COVID-19 Variant

The World Health Organization (WHO) is monitoring a new COVID-19 subvariant called XBB.1.16, which is now circulating in the U.S. after an initial surge of cases in India. The Times of India reported on Apr. 17 that there are now more than 60,300 active COVID-19 cases in the country. “In India, XBB.1.16 has replaced the other variants that are in circulation,” said Maria Van Kerkhove, WHO’s COVID-19 technical lead, during a Mar. 29 press conference. “So this is one to watch.”

XBB.1.16—which a group of scientists on Twitter nicknamed “Arcturus”—is one of more than 600 Omicron subvariants that are currently circulating. It’s similar to the XBB.1.5 variant that has dominated the U.S. throughout 2023 but is distinguished by a mutation in the spike protein that “may give it some additional growth advantages,” says Dr. Peter Hotez, co-director of the Texas Children’s Hospital Center for Vaccine Development. In lab studies, this additional mutation “shows increased infectivity, as well as potential increased pathogenicity,” Van Kerkhove said. “It has potential changes that we need to keep a good eye out on.”

Initially, XBB.1.16 was found primarily in India and Nepal, but it’s now circulating in 29 countries, according to the WHO’s latest report. The variant reached the U.S. in early April, where it’s now responsible for 7.2% of cases nationwide. As we’ve seen with past variants, things can change quickly, Hotez says. Take XBB.1.5, which accounted for less than a quarter of COVID-19 cases in the U.S. going into late December last year. “Over the ensuing week, XBB.1.5 became the dominant variant,” says Hotez. Now that the newer variant is in the U.S., “the question is whether XBB.1.16 is going to do the same, or whether the two are [closely related] enough.”

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Still, a new variant—even one ratcheting up case numbers—is not by itself cause for alarm. “We haven’t seen a change in severity in individuals or in populations,” Van Kerkhove said during the March press conference. Despite the increase in cases, reported hospitalizations and deaths in India have remained low, says Dr. Monica Gandhi, associate division chief at UC San Francisco’s Division of HIV, Infectious Diseases, and Global Medicine, and Indian health officials have responded to the new dominant variant by recommending additional boosters for people over 65 or who are immunocompromised and otherwise vulnerable. Right now, many countries have high population level immunity, so boosting those who are vulnerable at the outset of a wave of cases is a reliable and repeatable strategy, she says.

“Any new subvariant of Omicron, it’s not fazing us,” Gandhi says. “It doesn’t change [the WHO’s] baseline recommendation, which is to boost older and vulnerable people. There is a possibility that we get really unlucky and we get a worse variant, and then we would boost everyone, but the reason that we know this is not what XBB.1.16 is is that it’s causing the expected rise in cases without a high rise in hospitalizations.”

On Apr. 18, the U.S. Food and Drug Administration (FDA) released its latest vaccination guidelines, which cleared Americans ages 65 and older to receive a second bivalent booster shot this spring.

It’s been a year since WHO released their plan for how the world will get out of the emergency phase of the pandemic, which included three possible paths. Even with the emergence and spread of variants like XBB.1.16, Gandhi says that the world is still on the best-case scenario path, in which new variants continue to appear but don’t cause outbreaks with high occurrences of severe disease and hospitalizations.

As the virus continues to evolve, Hotez is most concerned about the “dismantlement” of independent tracking efforts that the U.S. has relied on to spot and monitor new variants before they become dominant. “I think we’re letting our guard down,” he says.