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(NEXSTAR) – The U.S. has a new dominant COVID variant, the latest data from the Centers for Disease Control and Prevention shows.

Health officials worldwide have been warning of XBB, a subvariant of omicron, since the fall. As of Friday, XBB’s spinoff, XBB.1.5, makes up roughly 40% of U.S. COVID cases, the CDC reports.

Currently, it is most prevalent in the Northeast, making up about 75% of COVID cases from Maine south to New York and New Jersey. The initial strain, XBB, makes up only about 4% of the nation’s COVID cases.

XBB was first detected in India in August and spread quickly through Southeast Asia, according to the World Health Organization. At the time, the WHO described XBB as “the most antibody-evasive SARS-CoV-2 variant identified to date.”

Dr. Armando Paez, the Chief of the Division of Infectious Diseases at Baystate Health in Massachusetts, told Nexstar’s WWLP that XBB can likely evade immunity from prior infections as well.

“So most likely, if you’re going to be exposed to it, you may get infected,” he said.

Since first being discovered, XBB has evolved and now has two subvariants, XBB.1 and XBB.1.5.

XBB.1.5 differs from its family members because it has a mutation that allows it to better bind to cells, Andrew Pekosz, a virologist for Johns Hopkins University, told CNBC. That ability to bind gives it a better chance of infecting people.

The XBB.1.5 subvariant so far hasn’t proven to be more severe than others, Dr. Barbara Mahon, director of the CDC’s proposed Coronavirus and Other Respiratory Viruses Division said during an interview with CBS News. Dr. Jay Varma of the Cornell Center for Pandemic Prevention and Response echoed those statements to PBS News Hour, adding that if you have received the vaccines and have been previously infected, “you have a similar level of protection” against this subvariant.

Studies have suggested, though, that it may not respond as well to antibody treatment as other variants and subvariants have. According to Varma, the monoclonal antibodies treatment “is no longer effective,” leaving healthcare providers with Paxlovid, “the one drug that’s widely available, as our effective treatment option.”

Mahon said the bivalent booster is expected to be as effective at protecting against XBB.1.5 as it has with other omicron subvariants.

Testing, however, may not be as effective at detecting XBB.1.5. On Thursday, the FDA warned an at-home saliva testing kit, DxTerity, has been found to “have significantly reduced sensitivity” to some omicron mutations, including the XBB, BA.2.75, and BN.1.

Medical professionals and the CDC continue to recommend wearing a mask, getting vaccinated, and staying home if you aren’t feeling well — even if you’re testing negative for COVID.

XBB.1.5 comes as Americans are already facing other respiratory illnesses like the flu, RSV, and even a potential outbreak of strep.

Nexstar’s Alix Martichoux contributed to this report.