3 Things to Know About XEC, the Dominant COVID Strain

[Originally published: Oct. 1, 2024; Updated: Dec. 20, 2024.]

Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.

Experts predicted that a highly transmissible coronavirus subvariant called XEC—one that spread persistently throughout the country in 2024—would become the dominant SARS-CoV-2 strain. In early December, it did just that, right on time for winter, when COVID-19 infections and hospitalizations tend to peak.

XEC is a subvariant of the SARS-CoV-2 Omicron strain, which has spawned multiple descendants since it surfaced in the U.S. in 2021. XEC first appeared in Germany in June 2024 before spreading rapidly throughout other parts of Europe.

The good news is that so far there is no evidence that XEC causes different symptoms or more severe disease than other recent strains—although a COVID infection is always a concern for certain people, including those who are older or immunocompromised.

In early December, XEC was responsible for an estimated 45% of COVID infections in the U.S.—making it the most common strain here (it rose from seventh place at the end of September), according to a Centers for Disease Control and Prevention (CDC) estimate at the time. It was closely followed by the KP3.1.1 strain, which accounted for 24% of infections.

“One reason for the concern is that XEC has moved quickly enough to outpace the growth of all other SARS-CoV-2 variants in a few areas in Europe,” says Scott Roberts, MD, a Yale Medicine infectious diseases specialist. “The rate of infections from XEC they're seeing in some countries rose pretty quickly compared to previous variants in those same places.” By the end of September, XEC had been detected in at least 25 states in the U.S., he adds.

The XEC COVID strain is also rising at a time when people are getting the updated 2024-2025 COVID vaccines that became available in the U.S. in August. Dr. Roberts is encouraging everyone ages 6 months and older to get vaccinated with the updated shot for protection against the virus.

Dr. Roberts discussed three things that everyone should know about the XEC strain.

One distinguishing factor is that XEC is a recombinant strain—a hybrid resulting from a merger and rearrangement of two pre-existing COVID subvariants: Omicron sublineages KP.3.3 (a descendant of the FLiRT variants) and KS.1.1.

Such a merger can occur when one person is infected with two different strains. “So, instead of this slow marching along—one mutation at a time—that we’ve been seeing with the SARS-CoV-2 variants, this has the potential to create a big jump, because it combines full segments of two genomes into one new genome,” says Dr. Roberts.

But a “big jump” remains to be seen and is still probably unlikely, he adds.

Another difference is that the XEC strain has at least one new mutation beyond those two strains in its spike protein, although how this might affect a person who is infected isn’t clear at this point. But Omicron and its subvariants have generally caused milder disease than strains that surfaced early in the pandemic, such as Alpha and Delta—and the symptoms haven’t changed, Dr. Roberts says.

According to the CDC, symptoms still include (but are not limited to) cough, congestion or runny nose, diarrhea, fever or chills, shortness of breath, and loss of taste or smell. The symptoms may start out as mild, but in some people they can become more severe as the disease progresses. “I'm not aware of any changes in the behavior of the virus outside of the increased transmissibility,” Dr. Roberts says.

The vaccines are expected to work well against XEC and other variants that are rising and could be predominant in the future, such as MC.1., according to the CDC. The MC.1 strain is a descendent of the KP.3.1.1 variant, and while it only accounts for 5% of cases, it has caused a growing number of infections as well.

It’s impossible to guarantee a 100% match between a vaccine and a circulating variant when a virus is constantly mutating, Dr. Roberts says. But the newly updated Pfizer and Moderna mRNA vaccines, designed to target KP.2 (the dominant COVID strain this past spring), and the latest Novavax vaccine that targets an earlier strain— JN.1—should also provide protection against XEC, he adds.

“Even though XEC is recombinant, it's composed of two Omicron subvariants that were expected to be addressed by the updated vaccines,” Dr. Roberts says. “From that standpoint, although this new variant might diminish the immunity the vaccines provide by a little bit, I'm optimistic that we're still going to have some degree of protection from both recent infections and updated vaccines.”

He also points out that JN.1 had 30 mutations distinguishing it from the XBB strain, which last winter’s updated vaccine was made to protect against. “That vaccine still offered good protection against JN.1, even though it was very distinct,” Dr. Roberts says. “So, I would predict the same thing will happen now.”

Get the updated vaccine, says Dr. Roberts, adding that while the summer surge appears to be winding down, a winter wave of COVID cases is expected.

“Usually, the way these waves work is that we'll have six to eight weeks of diminished activity, and then it will pick up again,” he says. “I always recommend getting vaccinated by October at the latest, so you’ll have peak immunity during the holidays, when there's a lot of travel and mingling with family, often indoors where the virus can spread more easily.”

He also recommends taking the usual precautions to avoid a COVID infection, including staying away from people who are sick, wearing a mask in crowded indoor spaces, and following other recommendations provided by the CDC.

If you do test positive for COVID, antivirals such as Paxlovid are recommended. Paxlovid is available by prescription to anyone ages 12 and up, depending on their risk of severe illness from COVID, among other factors. It is most effective when taken within the first few days of developing symptoms.