COVID-19 updates: Anti-vaccine protesters halt vaccinations at Dodger Stadium

Dr. Anthony Fauci, the top infectious disease expert in the United States, said he believes a new, more contagious variant of the novel coronavirus that was first detected in the United Kingdom is also more virulent.

When British scientists first studied the mutation "intensively" on a case-by-case basis, Fauci said they initially thought the variant was more transmissible but not necessarily more likely to cause serious illness or death.

"When they went and became more gradual and looked at the data, they became convinced that it is in fact a bit more virulent, namely making it more difficult when you get to the point of serious disease and even death. So I believe their data. I haven't seen all of it, but from what I've heard I believe the data," Fauci, director of the National Institute of Allergy and Infectious Diseases, told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

ABC News has learned that U.S. President Joe Biden plans to impose a ban on most non-U.S. citizens entering the country who have recently been in South Africa as well as reinstate an entry ban on almost all non-U.S. citizens from Brazil, Ireland, the United Kingdom and 26 other European nations that allow travel across open borders. The move comes as a new, more contagious variants of the novel coronavirus have emerged out of South Africa, Brazil and the U.K. Some researchers worry that the mutations could reduce the potency of the two COVID-19 vaccines currently being used in the U.S.

Fauci, who is Biden's chief medical adviser, said he believes the South Africa travel ban was "prudent" and "the right decision," though he admitted "there's always a possibility and even a likelihood of some slippage."

"Right now, even though our surveillance isn't as comprehensive as we'd like it to be yet, it doesn't appear that this particular mutant is in the United States -- although it well may be," he said. "But if you have a free inflow of people from a country in which that mutant is clearly dominant -- I talk to my colleagues often in South Africa, it clearly is dominant there -- I believe the travel ban will be important, in addition to having a situation where anybody coming into the country now is going to be required to have a negative test before they even get on the plane, when they land to have a degree of quarantine as well as another test.

Fauci expressed hope that the vaccine shortages and delays in many communities across the U.S. will be corrected "within a very reasonable period of time," while cautioning, "it's going to take a little time."

"We've got to pull out all the stops," he said. "We've got to get into the trenches and figure out exactly at that local level what's going on and how to fix it."

He said "getting the vaccine situation on track" is Biden's "highest priority."

"The president has said it publicly, but in a private session, just sitting down with him and with the medical team, he's very serious. He said: 'Science is going to rule. We're going to go by the facts. We're going to go by the evidence. We're going to go by the data,'" Fauci recalled. "So the idea that the president himself sits down with you, says, 'I want science to rule, go out there and do everything we need to do to get it done,' just is really very refreshing."

U.S. President Joe Biden will impose a ban on most non-U.S. citizens entering the country who have recently been in South Africa as well as reinstate an entry ban on almost all non-U.S. citizens from Brazil, Ireland, the United Kingdom and 26 other European nations that allow travel across open borders, a White House official told ABC News.

The new restrictions are expected to go into effect on Saturday, the official said.

Reuters was first to report the news.

The move comes as a new, more contagious variants of the novel coronavirus have emerged out of South Africa, Brazil and the U.K. Some researchers worry that the mutations could reduce the potency of the two COVID-19 vaccines currently being used in the U.S.

ABC News' Molly Nagle contributed to this report.

A ban on almost all incoming flights went into effect in Israel on Monday.

The ban will last until Jan. 31, according to a joint statement from the Israeli Prime Minister's Office, the Ministry of Health and the Ministry of Transport.

The measure applies to all incoming flights except for cargo planes, aerial firefighting and flights for emergency medical evacuation. There is also a "temporary restriction" on permits for operating Israeli airlines, according to the statement.

Meanwhile, people are now only allowed to fly out of Israel for medical treatment, judicial proceedings to which the person is a party or must participate in, or the funeral of a close relative. The measure also applies to private Israeli planes, according to the statement.

The Israeli government previously announced it is extending the country's lockdown to the end of the month amid a spike in COVID-19 infections, and that travelers are only allowed to board a flight to Israel on presentation of a negative COVID-19 test during the 72 hours preceding travel.

Dr. Anthony Fauci, the nation's leading infectious disease expert, opened up about his experience working with former U.S. President Donald Trump in an interview with The New York Times that was published Sunday.

When COVID-19 began to rapidly spread in the northeastern part of the country last year, particularly in New York City, Fauci said Trump had "almost a reflex response" to try to "minimize" the situation.

"I would try to express the gravity of the situation, and the response of the president was always leaning toward, 'Well, it's not that bad, right?' And I would say, 'Yes, it is that bad,'" Fauci, director of the National Institute of Allergy and Infectious Diseases, told the newspaper. "It was almost a reflex response, trying to coax you to minimize it. Not saying, 'I want you to minimize it,' but, 'Oh, really, was it that bad?'"

Fauci, who was a key member of the Trump administration's coronavirus task force, said another thing that made him "really concerned" was the former president taking input from non-experts on unproven methods to treat COVID-19, like hydroxychloroquine.

"It was clear that he was getting input from people who were calling him up, I don’t know who, people he knew from business, saying, 'Hey, I heard about this drug, isn't it great?' or, 'Boy, this convalescent plasma is really phenomenal,'" Fauci told the newspaper. "And I would try to, you know, calmly explain that you find out if something works by doing an appropriate clinical trial; you get the information, you give it a peer review. And he’d say, 'Oh, no, no, no, no, no, no, no, this stuff really works.'"

"He would take just as seriously their opinion -- based on no data, just anecdote -- that something might really be important," Fauci added. "That’s when my anxiety started to escalate."

When the leadership of the White House coronavirus task force changed hands last February, with U.S. Vice President Mike Pence coordinating the government's response and Trump at the podium taking questions from reporters during the press briefings, Fauci said it went from "the standard kind of scientifically based, public-health-based meetings" to "the anecdotally driven situations, the minimization, the president surrounding himself with people saying things that didn’t make any scientific sense."

"Then I started getting anxious that this was not going in the right direction," he told the newspaper. "We would say things like: 'This is an outbreak. Infectious diseases run their own course unless one does something to intervene.' And then he would get up and start talking about, 'It’s going to go away, it’s magical, it’s going to disappear.'"

That's when Fauci said it became clear to him that he needed to speak up, even if it meant contradicting the president.

"He would say something that clearly was not correct, and then a reporter would say, 'Well, let’s hear from Dr. Fauci.' I would have to get up and say, 'No, I’m sorry, I do not think that is the case,' he told the newspaper. "It isn’t like I took any pleasure in contradicting the president of the United States. I have a great deal of respect for the office. But I made a decision that I just had to. Otherwise I would be compromising my own integrity, and be giving a false message to the world. If I didn’t speak up, it would be almost tacit approval that what he was saying was OK."

This upset Trump's "inner circle," Fauci said.

"That’s when we started getting into things I felt were unfortunate and somewhat nefarious -- namely, allowing Peter Navarro to write an editorial in USA Today saying I’m wrong on most of the things I say," he told the newspaper. "Or to have the White House press office send out a detailed list of things I said that turned out to be not true -- all of which were nonsense because they were all true. The very press office that was making decisions as to whether I can go on a TV show or talk to you."

Fauci said there were a couple times where Trump even called him personally to say, "Hey, why aren’t you more positive? You’ve got to take a positive attitude. Why are you so negativistic? Be more positive."

Fauci said he and his family have received death threats, beginning last March, and that his wife once suggested he consider quitting.

"But I felt that if I stepped down, that would leave a void. Someone’s got to not be afraid to speak out the truth," he told the newspaper. "Even if I wasn’t very effective in changing everybody's minds, the idea that they knew that nonsense could not be spouted without my pushing back on it, I felt was important. I think in the big picture, I felt it would be better for the country and better for the cause for me to stay, as opposed to walk away."