There’s a new Covid variant in town.

The strain, known as XEC, is gaining a foothold in the United States, accounting for an estimated 5.7% of new casesin the past two weeks,the Centers for Disease Control and Prevention said Friday.

“We don’t know if it will have legs and soar up in the charts,” said Dr. Peter Chin-Hong, a professor of medicine and an infectious disease specialist at the University of California, San Francisco. “But it has the features that could make it the one to watch.”

XEC caught the eye of experts as it spread quickly in parts of Europe in recent weeks. Still, a lot remains unknown about the strain.

“Once again, Covid is showing that it is not finished mutating, and we’re still seeing the limitations of our vaccines in controlling it,” said Rick Bright, an immunologist and the former director of the Biomedical Advanced Research and Development Authority, part of the Department of Health and Human Services.

Chin-Hong said the variant is a so-called recombinant of two other strains — KS.1.1 and KP.3.3 — meaning that genetic information was exchanged between them to form a third strain, XEC, Chin-Hong said.

While the CDC says that overall levels of Covid remain high in the U.S., it appears that levels of the virus in wastewater are trending downward after peaking in August.

Is the XEC variant more contagious?

Bill Hanage, an epidemiologist and the associate director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health, said there’s no evidence yet that the XEC variant is much more transmissible than what’s currently circulating.

“To be convinced that it was more contagious, I would want to have seen it taking off in more places by now,” Hanage said. “It’s not clear from what we’ve seen that it’s that great in comparison with its immediate peers,” referring to the other variants in circulation.

“It’s just the latest kid on the block,” he added. “and we’re going to see more kids on the block.”

XEC was initially detected in May, but so far cases haven’t been doubling at an alarming rate, Hanage said.

“It’s not a doubling time which is especially disturbing,” he said.

Marc Johnson, a virologist and professor of molecular microbiology and immunology at the University of Missouri School of Medicine, said the XEC variant really took off in Germany and has since spread to neighboring countries, including the Netherlands and France.

Since the variant has only gotten a strong foothold in a handful of countries, however, it’s possible that the reason for its success in those places is simply that it was the right virus in the right place at the right time, Johnson said.

This is a phenomenon known as the founder effect, Hanage said, adding that it’s not yet clear if the virus itself is more contagious or if its spread was driven by something else, like an increase in summer travel.

“While I’m sure that we’re going to see XEC here and it’s going to be significant, I wouldn’t be particularly laying money on it taking over completely, even by the winter months,” he said.

There’s also no evidence yet that the strain will cause more severe illness, Hanage said, adding that it’s unlikely to do so given its genetic mutations.

Will the vaccines still work against XEC?

XEC, like countless others before it, is a part of the omicron family. The variant is closely related to KP.2, the strain targeted by the updated vaccines from Pfizer and Moderna. And both XEC and KP.1 are descendants of the JN.1 strain, targeted by the updated Novavax vaccine.

“I think we have good reason to think that there will be substantial protection from the boosters that are available,” Hanage said.

The part of the virus that antibodies bind to, known as the spike protein, is similar between these strains.

“I’m glad it doesn’t look terribly different,” Chin-Hong said. “It’s not like a completely new branch. So that’s why I think the vaccine is still generally a decent match.”

Johnson said that KP.3.1.1, the dominant variant during this summer’s surge in cases, is also similar to the XEC variant, meaning those who were recently infected may have enhanced immunity from the latest strain.

Bright, however, cautioned that clinical data is ultimately needed to determine how well the updated vaccines work against the XEC variant.

“Until we start seeing some clinical data on how well the mRNA and Novavax vaccines provide immunity from this new variant, we really cannot see with confidence how bad this season will be,” Bright said.