Three years into the pandemic, the coronavirus continues to impress virus experts with its swift evolution.

A young version, known as XBB.1.5, has quickly been spreading in the United States over the past few weeks. As of Friday, the Centers for Disease Control and Prevention estimated that it made up 72% of new cases in the Northeast and 27.6% of cases across the country.

The new subvariant, first sampled in autumn in New York state, has a potent array of mutations that appear to help it evade immune defenses and improve its ability to invade cells.