Sweeping new federal recommendations announced on Monday for Americans to sharply limit their activities appeared to draw on a dire scientific report warning that, without action by the government and individuals to slow the spread of coronavirus and suppress new cases, 2.2 million people in the United States could die.

To curb the epidemic, there would need to be drastic restrictions on work, school and social gatherings for periods of time until a vaccine was available, which could take 18 months, according to the report, compiled by British researchers. They cautioned that such steps carried enormous costs that could also affect people’s health, but concluded they were “the only viable strategy at the current time.”

That is because different steps, intended to drive down transmission by isolating patients, quarantining those in contact with them and keeping the most vulnerable apart from others for three months, could only cut the predicted death toll by half, the new report said.

The White House guidelines urged Americans to avoid gatherings of more than 10 people. That is a more restrictive stance than recommendations released on Sunday by the U.S. Centers for Disease Control and Prevention, which said that gatherings should be limited to 50.