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The coronavirus mutant that is now dominant in the United States is a member of the omicron family but scientists say it spreads faster than its omicron predecessors, is adept at escaping immunity and might possibly cause more serious disease.

Why? Because it combines properties of both omicron and delta, the nation’s dominant variant in the middle of last year.

A genetic trait that harkens back to the pandemic’s past, known as a “delta mutation,” appears to allow the virus “to escape pre-existing immunity from vaccination and prior infection, especially if you were infected in the omicron wave,” said Dr. Wesley Long, a pathologist at Houston Methodist in Texas. That’s because the original omicron strain that swept the world didn’t have the mutation.

This genetic change is bad news for people who caught the original omicron and thought that made them unlikely to get COVID-19 again soon. Although most people don’t know for sure which variant caused their illness, the original omicron caused a giant wave of cases late last year and early this year.