What is XEC, the new COVID-19 variant?
XEC is on track to become the new dominant COVID-19 strain this winter. Yale and New Haven experts weigh in on why you should still get the new vaccine even though it may be less effective at neutralization because it is a recombinant variant.
Zoe Berg
As of Oct. 4, the XEC COVID-19 variant has spread to at least 33 countries and has been reported in at least 29 states across the U.S. Experts who spoke with the News said that the new booster shot still neutralizes XEC but to a lesser degree when compared to other strains.
According to the Centers for Disease Control and Prevention, XEC accounted for an estimated 6.0 percent of new cases in the past two weeks. While recent preprint studies indicate that the new COVID-19 booster still neutralizes the strain, it has slightly reduced efficacy in doing so compared to other variants. Experts say that XEC’s special characteristics, including increased transmissibility and unique mutations, come from its being a recombinant strain — meaning that instead of being a result of a typical mutation, XEC is a combination of previous subvariants of the virus.
“Recombination in theory, is a bit scarier [compared to typical mutations] because you could have a complete reassortment of the viral genome where all of a sudden it picks up 30 mutations from a very distantly related virus and can now fully bypass our immune defenses and lead to a big surge,” said Scott Roberts, the associate medical director of Infection Prevention at Yale New Haven Health.
According to Roberts, the classic variance of viruses such as COVID comes from the journey a virus takes from person to person. As it passes between people, it starts picking up mutations, and over time the result is a slow crawl towards something different. Throughout this period, the virus can remain harmless until it reaches someone who has not received an updated vaccination or has not been recently infected. If the virus has since become different enough from what their immune system recognizes, it leads to an infection.
In contrast, recombinant variants pick up chunks of genetic material from two different viruses, effectively merging them, rather than evolving over time. This mechanism allows recombinant variants to potentially become more dangerous, for they can quickly alter in ways that make them harder for the immune system to recognize and combat. In XEC’s case, it is a hybrid of two previously discovered Omicron sub variants: KS.1.1 and KP.3.3.
However, Roberts says that this recombinant variant may not be a cause for great concern as it is a hybrid of two variants from the same branch of COVID-19.
“The good news is that this is a recombinant combination of two Omicron sub variants, which we’ve really been involved with since 2022,” Roberts said. He explains that the shift from previous Omicron variants to XEC is more manageable than the shift from the Delta variant to Omicron in 2021. He added that XEC is not “a new Greek letter or anything where it’s an out-of-left-field thing we should be worried about.”
The New Haven Health Department wrote to the news sharing similar conclusions.
“So far, we’ve seen that XEC is more transmissible, but it doesn’t appear to cause more severe illness than previous Omicron variants,” the health department wrote. “It is showing a growth advantage over other variants, meaning it spreads faster through the population than other variants.”
Roberts also told the News that, although XEC has not proven to be a significant concern to people without preexisting health conditions, recombinant variants disproportionately affect the immunocompromised community.
“We had a profoundly immunocompromised patient who had COVID for over a year and their mutation rate was almost double that of a healthy person,” said Roberts. “So this patient individually developed three variants on their own just because their immune system couldn’t clear the virus, so it led to this really rapid mutation.”
Roberts says that we live in a “pretty immunocompromised society” and that makes getting the vaccine especially important.
Albert Ko, a professor of public health and epidemiology, explained it is important for individuals who live with people at risk to get vaccinated.
“We’re living with parents and grandparents who may have comorbidities, which includes just being old,” he said. “That’s a good reason why we should get vaccinated. The updated vaccine that has just come out in the last month will help prevent our grandparents or our parents from getting severe COVID and being hospitalized from COVID.”
Ko refers to herd immunity from vaccines, which helps protect immunocompromised individuals. The phenomenon occurs when a large portion of a population becomes immune to an illness, either through vaccination or previous infection, reducing its spread. This protects those who are not immune because the disease has fewer chances to transmit within the community, which is especially important for a highly transmissible virus like XEC.
According to Roberts, due to the recombinant nature of XEC, the new COVID-19 vaccine will likely still be effective but will have diminished protection. This vaccine targets different strains of Omicron variants but is not an exact match to XEC, especially since it is a combination of strains.
However, Roberts says that this imperfect match is typical, citing that even the annual flu vaccines are not perfect matches to the dominant circulating strain. This is because the vaccines are developed and manufactured in the summer to match the strain predicted to be circulating in the winter.
Referring to a preprint study posted earlier this week, Roberts emphasizes that everyone should get the new booster shot this winter, including those who have already received one last year.
“Critically, neutralization [from the new vaccine compared to the old one] increased in XEC,” Roberts said. “So the vaccine will work, but not as well as current circulating variants. I’m not too surprised by this, but the good news is that there is still at least some good immune response against XEC, and vaccination should be encouraged.”
According to Ko, neutralization is a laboratory test that tells us if antibodies from vaccinated individuals can prevent the virus from spreading to other cells in a test tube. For XEC, it neutralizes, but not as well as earlier strains.
Regardless, Ko agreed that getting the vaccine is still important because it can still “prevent people, especially those at high risk, from severe disease, hospitalization and death.”
Ko suggests everyone get tested if they are showing any kind of symptoms and “do the responsible thing, which would be to stay at home if you are sick because that’s when you’re most infectious. And then, follow the CDC recommendations” of staying home and away from other people until at least 24 hours after symptoms subdued.
Flu shots are free for all Yale faculty, staff, students and Yale Health members through on-site vaccination clinics.