WASHINGTON - The United States needs to move faster to immunize the public against the coronavirus, but efforts to accelerate beyond President Joe Biden's goal of 100 million shots in 100 days may be hindered by the lack of vaccine doses, according to Rochelle Walensky, the new director of the Centers for Disease Control and Prevention. 

"I think that the supply is probably going to be the most limiting constraint early on, and we're really hoping that after that first 100 days, we'll have much more production," Walensky said on "Fox News Sunday."

"We're really hoping we'll have more vaccine and that will increase the pace at which we can do the vaccinations," she said.

As officials push to boost inoculation efforts to help crush the pandemic, scientists are challenged by trying to understanding coronavirus variants. Anthony Fauci, the government's senior infectious-disease expert and Biden's chief medical adviser for the pandemic, said the coronavirus variant first detected in the United Kingdom is more deadly and spreads faster.

"We need to assume now what has been circulating dominantly in the U.K. does have an increase in what we call virulence to cause more damage, including death," Fauci said on CBS News's "Face the Nation."

On Saturday, the United States reported more than 1.3 million newly administered doses of coronavirus vaccines, the fifth day in a row the country has topped 1 million daily doses, according to data tracked by The Washington Post. That pace suggests that the country already is on track to meet the Biden administration's 100-day goal. The target was criticized by some who said it was unrealistic when Biden announced it in December, but it now seems less ambitious because of increased manufacturing certainty and a ramped-up inoculation pace in the last days of the Trump administration.

Biden defended the vaccination goal Thursday. "When I announced it, you all said it's not possible. Come on, give me a break, man. It's a good start."

Biden officials have since said the 100 million figure is their starting point, not a final goal.

The country appears to have avoided worst-case scenarios for a surge in the wake of holiday gatherings, but experts say the virus's threat could intensify with the emergence of new variants.

Up to 100 sites run by the Federal Emergency Management Agency could begin offering doses of coronavirus vaccines within the next month, an expansion of the federal government's role in fighting the pandemic.

People who have received their first vaccine dose can schedule their second shot up to six weeks later if they are not able to get one in the recommended time frame, according to updated guidance from the CDC. "We're just ensuring clinicians that if they can't do it at exactly 21 days or 28 days, that there's leeway or flexibility," said CDC spokeswoman Kristen Nordlund.

Speaking about the coronavirus variant now dominant in his country, British Prime Minister Boris Johnson said in a statement Friday that it "may be associated with a higher degree of mortality."

"We have every reason to believe them," Fauci said Sunday of Johnson's statement. He added: "We want to look at the data ourselves."

That variant of the virus is thought to be circulating at a relatively low level within the United States. The CDC warned less than two weeks ago that it probably will become the dominant strain in this country within a couple of months.

Fauci said the strain's greater virulence "is likely, but we can't say definitively," because the United States has not been conducting enough genomic testing to pin down the strains of positive test results.

"There's a lot of movement at the CDC to dramatically increase genomic surveillance," he said.

In her interview on "Fox News Sunday," Walensky said health officials are increasing their surveillance and study of variants and are monitoring their effect on the vaccines. She called their emergence a signal that "we need to get more vaccine out there."

Referring to supply constraints, Walensky said the Biden administration is working with manufacturers to address problems and increase production by March.

"One of the biggest problems right now is I can't tell you how much vaccine we have, and if I can't tell it to you, then I can't tell it to the governors, and I can't tell it to the state health officials," Walensky said.

That lack of data, she said, is making it harder for state leaders to plan vaccine distribution.

In the day before she took over at the CDC and in the few days since, Walensky has repeatedly said publicly that the goals for vaccination and the decisions about when to expand the groups eligible for it must square with the amount of vaccine doses available in the United States.

In an interview with NBC on Thursday, she suggested that top health officials in the Trump administration had been overly optimistic in predicting late last year that coronavirus vaccines could be widely available to the American public by late February or early March.

"I don't think late February we're going to have vaccine in every pharmacy in this country," Walensky said. She said the new administration would adhere to the 100-day goal, adding: "We ... also want to be very cognizant of the fact that after 100 days, there are still a lot of Americans who need vaccine, so we have our pedal to the metal to make sure we can get as much vaccine out there."

Fauci said Sunday that the pace of recent days may be difficult to maintain as more vaccinations occur outside controlled settings, such as hospitals and nursing homes, and more broadly in communities.

Biden's nominee for surgeon general, Vivek Murthy, called for investing in treatment strategies, testing and contact tracing, especially with the spread of coronavirus variants that are "likely to be more transmissible."

"The variants, they are really a shot across the bow. The virus is basically telling us that it's going to continue to change, and we have got to be ready for it," Murthy said Sunday on ABC News's "This Week." "The bottom line is, we're in a race against these variants, the virus is going to change, and it's up to us to adapt and to make sure that we're staying ahead."

Murthy also pushed back on criticism of Biden's goal of 100 million vaccinations, saying it is "not a ceiling. It's also a goal that reflects the realities of what we face, what could go right but also what could go wrong."

"I think President Biden fully understands there's a larger goal here, as we all do, which is that we've got to vaccinate as many Americans as possible," Murthy said. "And that's going to take a lot of work - work dispelling misinformation, working on the [vaccine] supply, increasing distribution channels."

In an interview on NBC News's "Meet the Press," White House Chief of Staff Ron Klain also defended the targets, calling it "a very bold and ambitious goal." He said it was just the administration's first target, "not our final goal."

"This country has never given 100 million shots in 100 days, so if we can do that, I think it would be quite an accomplishment," Klain said Sunday. "But obviously, we're not going to stop there. I mean, 100 million people - 100 million shots - is a bold, ambitious goal, but we need to keep going after that."

- - -

The Washington Post's Cat Zakrzewski contributed to this report.