WASHINGTON (Reuters) — Herd immunity against the novel coronavirus could require vaccination rates approaching as high as 90%, Dr. Anthony Fauci, the most prominent U.S. infectious disease expert, said in an interview published on Thursday.

More than 1 million Americans have received a first dose of a vaccine since Dec. 14, according to the U.S. Centers for Disease Control, or only about 0.3% of the population.

Fauci, who is advising both President Donald Trump and President-elect Joe Biden on the pandemic, acknowledged that he had incrementally increased his estimates from earlier in the year, when he tended to say only 60% to 70% would need to be inoculated for herd immunity to be reached.

“We need to have some humility here,” Fauci told the New York Times. “We really don’t know what the real number is. I think the real range is somewhere between 70 to 90 percent. But, I’m not going to say 90 percent.”

His comments came as the country marks grim new daily milestones while contending with the world’s deadliest outbreak: it reported more than 3,000 deaths for the second consecutive day on Wednesday.

That same day, more Americans flew than on any other day since the pandemic emerged in March, with 1,191,123 passengers passing through airport checkpoints, according to data from the U.S. Transportation Security Administration.

The data suggested many were disregarding public health experts’ advice to avoid traveling to celebrate Christmas Day on Friday. Fauci and other experts say social distancing will be required deep into 2021 as vaccines are slowly rolled out.

The number of travelers was down from 2019, when 1,937,235 flew on Dec. 23. Wednesday’s traffic exceeded the previous pandemic-era high set on Nov. 29, the Sunday after the Thanksgiving holiday, when 1,176,091 people passed through TSA checkpoints, preceding new surges in coronavirus cases in many states.

Health care workers, elderly nursing home residents, elected officials and firefighters are among those receiving the vaccines first. Most Americans have been told it could be six months or more before they are eligible for the shots.

Fauci, who was appointed director of the National Institute of Allergy and Infectious Diseases in 1984, said in the interview that he had become more willing to reveal his beliefs as polls show Americans were becoming somewhat less skeptical about the new vaccines. The more infectious a disease is, the higher the rate of vaccination is required to reach a threshold of herd immunity, in which its spread is contained.

“When polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent,” Fauci, who turned 80 on Thursday, told the Times.

“Then, when newer surveys said 60 percent or more would take it, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85.'”