The coronavirus pandemic in the U.S. will face its darkest period so far over the next three to four months as cases continue to surge above 100,000 per day, a newly appointed coronavirus advisor to President-elect Joe Biden said on Monday.
"What America has to understand is that we are about to enter Covid hell," Dr. Michael Osterholm, director of the Center of Infectious Disease Research and Policy at the University of Minnesota, told CNBC's "Squawk Alley." "It is happening."
The U.S. is reporting a record-high weekly average of roughly 108,736 cases every day, growing more than 33% compared with a week ago, according to a CNBC analysis of data compiled by Johns Hopkins University.
The U.S. is nearing 10 million reported Covid-19 cases as of Monday, the most of any nation on the globe, according to Hopkins data. The coronavirus has killed at least 237,584 people in the country so far.
That figure has grown dramatically in recent weeks, sending the U.S. pandemic deeper into its third peak ahead of Thanksgiving and Christmas. After a summer of surging infections across America's Sun Belt states, the country was able to suppress the average number of cases to just above 30,000 a day in early September.
After Labor Day, Osterholm said, he warned the U.S. would see an "astronomical" increase in new cases. That prediction has come true in only a matter of weeks, he said, adding that "this number is going to continue to increase substantially."
"We have not even come close to the peak and, as such, our hospitals are now being overrun," Osterholm said. "The next three to four months are going to be, by far, the darkest of the pandemic."
And while Pfizer's early Covid-19 vaccine data showing more than 90% effectiveness is "a great, great finding," Osterholm said that more data is needed to understand how effective the vaccine is in preventing serious illness and death versus more mild symptoms.
"Until we have those pieces of information, we can't really know how much of a game changer this really is," he said.
On Monday, Biden's transition team announced the handful of members, including Osterholm, who will consult with state and local leaders to help steer the federal response once the president-elect takes office on Jan. 20.
The task force will be co-chaired by former Surgeon General Vivek Murthy, former Food and Drug Administration Commissioner David Kessler and Dr. Marcella Nunez-Smith of Yale University, according to the announcement.
Osterholm's concern about the looming winter months echo that of Biden, who also said on Monday that the U.S. is "still facing a very dark winter" while urging Americans to wear a face mask.
"It's clear that this vaccine, even if approved, will not be widely available for many months yet to come," Biden said. "The challenge before us right now is still immense and growing."