In interviews on Tuesday, some scientists argued that the variant’s rapid spread requires more vigorous mitigation measures.

Some expressed frustration and alarm about what they described as a timid public health response, and bemoaned the apparent lack of will among politicians and society at large for more aggressive steps…

“I still can’t quite wrap my head around how quickly this is moving,” said Joseph Fauver, a genomic epidemiologist at the University of Nebraska Medical Center. “I think it’s going to be really bad. I don’t know how else to put it.”

It is not yet clear whether the variant causes milder illness than earlier variants. But there is a concern among some scientists that the notion has gained wide circulation and that the pandemic-weary public has let down its guard.

“This is an incredibly contagious pathogen, and we don’t know yet its impact on severity and death,” said Galit Alter, an immunologist and virologist affiliated with the Ragon Institute of Massachusetts General Hospital, M.I.T. and Harvard.