France's world-renowned Pasteur Institute announced Monday that it is stopping development of one of its COVID-19 vaccine candidates, after finding the shot to be less effective than hoped.

The vaccine candidate, based on an existing measles vaccine, was being developed with American pharmaceutical company Merck and was put into phase 1 clinical trials last August. A review of interim results from the trials showed the shot "was well tolerated but produced immune responses that were inferior to those observed in people who had recovered naturally and to those observed in the authorized vaccines," according to a statement from the Pasteur Institute, which is located in Paris and is named after French scientist Louis Pasteur who created successful vaccines against rabies and anthrax in the 19th century.

The decision to abandon development of the drug does not have any impact on the institute's continued research of two other prospective vaccines, which are "based on different methodologies" and are not yet ready for clinical trials, according to the statement.

"The Pasteur Institute will continue to develop other vaccine candidates that have reached the end of the preclinical phase, and maintain its strong scientific mobilization to fight the COVID-19 epidemic," the institute said.

ABC News' Ibtissem Guenfoud contributed to this report.

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.