“It’s still early days, but increasingly, data is starting to trickle in, suggesting that omicron is likely to outcompete delta in many, if not all, places,” said Dr. Jacob Lemieux, who monitors variants for a research collaboration led by Harvard Medical School. “Certainly, it’s potentially alarming.”

But others said Monday it’s too soon to know how likely it is that omicron will spread more efficiently than delta, or, if it does, how fast it might take over.

“Especially here in the U.S., where we’re seeing significant surges in delta, whether omicron’s going to replace it I think we’ll know in about two weeks,” said Matthew Binnicker, director of clinical virology at Mayo Clinic in Rochester, Minn.