Americans should now assume there's already more contagious variants of the novel coronavirus circulating in their communities, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

"I think we should be treating every infection as if it's a variant," Walensky told ABC News chief anchor George Stephanopoulos in an interview Friday on "Good Morning America."

"That is the way we're going to control this pandemic," she added. "Quite honestly, we know that these viruses are going to mutate. They generally mutate to the advantage of the virus and that's how we get these more dominant strains."

Walensky's remarks come a day after the United States confirmed its first cases of the B1351 variant, which was first identified in South Africa and has since spread to dozens of other countries.

"We had always been worried that they were here and we hadn't yet detected them," she said.

The B1351 variant was discovered in two people in South Carolina who were not in contact with one another and haven't traveled recently, which concerns Walensky.

"So the presumption is here that they became infected from other people in the community and that there's community spread of this variant," she said.

Walensky explained that it "takes a while" for scientists to detect a variant.

"From the time of symptoms to somebody getting a test to that test being positive and to us being able to sequence it, that turnaround time could be up to 10 to 14 days," she said.

Although the CDC has "done an enormous amount of scaling up of our surveillance of these variants," Walensky said researchers are essentially starting from the ground up because "there has not been a public health infrastructure to track these variants."

"There has not been money, resources to be able to do mass sequencing at the level of infection that we have in this country right now," she said. "That is part of the American Rescue Plan, is to be able to use resources to finance a mass scale-up of surveillance for these variants."

There are concerns that the variants wield increased transmissibility and mortality, or that existing treatments and vaccines won't work as well against them.

"The current vaccines we're still studying against these variants," Walensky said. "What I will say though is we have a 95% efficacious vaccine against the current strain. Even if we have some diminution of that efficacy against the South Africa strain, I still think we need to really go ahead, push the vaccination, because this just is still yet another tool in our toolbox to fight this pandemic."

Russia said Friday it will be ready to supply Europe with enough doses of its COVID-19 vaccine, Sputnik V, for 50 million people in the second quarter of this year.

The Russian Direct Investment Fund (RDIF), which is responsible for worldwide marketing of the vaccine, announced via Twitter that 100 million doses can be provided to the European Union -- pending regulatory approval -- once most of Russia's population has been vaccinated.

After being developed by the state-run Gamaleya Research Institute of Epidemiology and Microbiology in Moscow, Sputnik V was controversially registered by the health ministry in August before starting crucial Phase 3 trials, with Russia declaring itself the first in the world to register a COVID-19 vaccine.

The RDIF said the vaccine is now registered in 15 countries and that documents have been submitted to the European Medicines Agency (EMA) for "rolling review," which would mean that the drug regulator is reviewing clinical trial data on a rolling basis. However, last week, the EMA said in a statement that "currently Sputnik V is not undergoing a rolling review."

ABC News' Alina Lobzina contributed to this report.

Mexico now has the third-highest death toll from COVID-19 in the world.

According to real-time data compiled by Johns Hopkins University, Mexico has registered 155,145 fatalities from the disease since the pandemic began, overtaking India's count of 154,010 deaths.

Mexico, a country of 127 million people, has confirmed more than 1.8 million cases of COVID-19. Whereas India, home to some 1.3 billion, has confirmed over 10.7 million cases, the second-most in the world, according to Johns Hopkins data.

There were 164,665 new cases of COVID-19 confirmed in the United States on Thursday, according to a real-time count kept by Johns Hopkins University.

Thursday's case count is far less than the country's all-time high of 300,282 newly confirmed infections on Jan. 2, Johns Hopkins data shows.

An additional 3,872 fatalities from COVID-19 were registered nationwide on Thursday, down from a peak of 4,466 new deaths on Jan. 12, according to Johns Hopkins data.

COVID-19 data may be skewed due to possible lags in reporting over the Martin Luther King Jr. Day holiday weekend.

A total of 25,766,735 people in the U.S. have been diagnosed with COVID-19 since the pandemic began, and at least 433,195 have died, according to Johns Hopkins data. The cases include people from all 50 U.S. states, Washington, D.C., and other U.S. territories as well as repatriated citizens.

Much of the country was under lockdown by the end of March as the first wave of pandemic hit. By May 20, all U.S. states had begun lifting stay-at-home orders and other restrictions put in place to curb the spread of the novel coronavirus. The day-to-day increase in the country's cases then hovered around 20,000 for a couple of weeks before shooting back up over the summer.

The numbers lingered around 40,000 to 50,000 from mid-August through early October before surging again to record levels, crossing 100,000 for the first time on Nov. 4, then reaching 200,000 on Nov. 27 before topping 300,000 on Jan. 2.

So far, the U.S. Food and Drug Administration has authorized two COVID-19 vaccines for emergency use -- one developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech, and another developed by American biotechnology company Moderna and the National Institute of Allergy and Infectious Diseases. More than 24 million vaccine doses have been administered nationwide, according to the latest data from the Centers for Disease Control and Prevention.

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."