Three years after the novel coronavirus emerged, a new variant, XBB.1.5, is quickly becoming the dominant strain in parts of the United States because of a potent mix of mutations that makes it easier to spread broadly, including among those who have been previously infected or vaccinated. XBB.1.5, pegged by the World Health Organization as “the most transmissible” descendant yet of the omicron variant, rose from barely 2 percent of U.S. cases at the start of December to more than 27 percent the first week of January, according to new estimates by the Centers for Disease Control and Prevention. More than 70 percent of cases in the Northeast are believed to be XBB.1.5.

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