CNN  — 

Dr. Anthony Fauci on Sunday expressed concern that the worst may still come in America’s battle against Covid-19, agreeing with President-elect Joe Biden’s recent assessment that the “darkest days” in fighting the virus lie ahead.

“And the reason I’m concerned and my colleagues in public health are concerned also is that we very well might see a post-seasonal, in the sense of Christmas, New Year’s, surge, and, as I have described it, as a surge upon a surge, because, if you look at the slope, the incline of cases that we have experienced as we have gone into the late fall and soon-to-be-early winter, it is really quite troubling,” Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN’s Dana Bash on “State of the Union.”

“We are really at a very critical point. … So I share the concern of President-elect Biden that as we get into the next few weeks, it might actually get worse.”

The comments from the nation’s top infectious disease expert come as the US records its deadliest month since the coronavirus pandemic’s start – with more than 63,000 Americans having died from the virus in December so far. More than 1 million people were screened in airports Saturday despite warnings from health officials to not travel for the Christmas holiday and the number of coronavirus cases continue to swell across the country.

Last week, Biden warned the “darkest days” in the battle against the virus “are ahead of us, not behind us,” and urged Americans to prepare themselves for the struggle while criticizing President Donald Trump’s handling of the pandemic.

“One thing I promise you about my leadership during this crisis: I’m going to tell it to you straight. I’m going to tell you the truth. And here’s the simple truth: Our darkest days in the battle against Covid are ahead of us, not behind us,” Biden, who has received a coronavirus vaccine, said in remarks at the time.

“So we need to prepare ourselves, to steel our spines. As frustrating as it is to hear, it’s going to take patience, persistence and determination to beat this virus. There will be no time to waste in taking the steps we need to turn this crisis around,” he said.

Fauci on Sunday again recommended that Trump, who has not yet received a vaccine, get vaccinated.

While the doctor, Biden and Vice President Mike Pence are among the notable government officials who have publicly received the vaccine, the President, who previously contracted the virus, has said he will be inoculated “at the appropriate time.” A White House official previously told CNN that Trump will be vaccinated when it is recommended by the White House medical team.

“The decision of whether he gets vaccinated is up to him and to the White House physician who is a person I know is a very competent physician,” Fauci said. “My recommendation, and I’ve said this before, is that I would get him vaccinated. He is still the President of the United States. A critical person.”

In its clinical guidance for the coronavirus vaccines authorized for emergency use in the United States, the US Centers for Disease Control and Prevention said the vaccines should be offered to people previously infected with coronavirus, as Trump was in early October. It noted that vaccination could be delayed somewhat, since reinfection is uncommon in the 90 days after initial infection.

However, there’s no safety or efficacy data for the vaccines in people who were treated for Covid-19 with monoclonal antibodies or convalescent plasma; Trump’s treatment for Covid-19 included the monoclonal antibody cocktail made by Regeneron. The CDC’s guidance said “vaccination should be deferred for at least 90 days, as a precautionary measure until additional information becomes available, to avoid interference of the antibody treatment with vaccine-induced immune responses.”

Herd immunity estimates adjusted

Fauci, asked about a New York Times report that he had been moving the goalposts on his estimate for what percentage of the population would need to be vaccinated against the virus in order to achieve herd immunity, told Bash that “the range is going to be somewhere between 70 and 85%.”

“We have to realize that we have to be humble and realize what we don’t know,” he said. “These are pure estimates and the calculations that I made, 70 to 75%, it’s a range. The range is going to be somewhere between 70 and 85%.”

The doctor added that the reason he first said 70 to 75% and then brought the figure up to 85% – an adjustment he deemed “not a big leap” – “was really based on calculations and pure extrapolations from measles.”

“So, I made a calculation that Covid-19, SARS-CoV-2, is not as nearly as transmissible as measles, measles is the most transmissible infection you can imagine,” he said. “So I would imagine that you would need something a little bit less than the 90%, that’s where I got to the 85.”

Everyone, Fauci said, has to be “honest and humble, nobody really knows for sure, but I think 70 to 85% for herd immunity for Covid-19 is a reasonable estimate, and in fact, most of my epidemiology colleagues agree with me.”

CNN’s Naomi Thomas and Jamie Gumbrecht contributed to this report.