President-elect Joe Biden on Tuesday sharply criticized the Trump administration over the pace of vaccinations for COVID-19, as prominent public health experts raised questions about the speed by which the federal government was getting shots into Americans’ arms.

In remarks in Delaware Tuesday afternoon, Biden accused the outgoing administration of being “far behind” in delivering what’s needed and what was promised, saying at the current rate, if would take years, not months, for all Americans to get vaccinated.

“A few weeks ago, the Trump administration suggested that 20 million Americans could be vaccinated by the end of December. With only a few days left in December, we’ve only vaccinated a few million so far. And the pace of the vaccination program is moving now, as it — if it continues to move as it is now, it’s going to take years, not months, to vaccinate the American people,” Biden warned.

The Trump administration swiftly pushed back, insisting that its plan to deliver first doses for 20 million Americans had only slipped into the first week of January, and that it was up to the states to ensure the injections were delivered in a timely manner.

“These doses are being distributed at states’ direction to the American people as quickly as they are available and releasable, and the rapid availability and distribution of so many doses — with 20 million first doses allocated for distribution just 18 days after the first vaccine was granted emergency use authorization — is a testament to the success of Operation Warp Speed,” said Michael Pratt, chief communications officer for the OWS program.

Biden reaffirmed his goal of administering 100 million shots in the first 100 days of his administration through a national strategy, saying the current vaccination effort, conducted at the state and local level, would need to be stepped up “five to six times” to reach his goal of 1 million shots a day.

“I’m going to move heaven and Earth to get us going in the right direction. I’m going to use my power under the Defense Production Act, when I’m sworn in, in order — and order private industry to accelerate the making of the materials needed for the vaccines as well as protective gear,” Biden said, outlining his own plans to combat COVID-19 as president, including a public education campaign to boost trust in the vaccine, particularly in communities of color.

But while Biden’s transition is working on plans to increase the number of vaccinations, the president-elect cautioned that the public should be prepared for the the monumental challenge ahead.

“This will take more time than anyone would like and more time than the promises from the Trump administration have suggested. This is going to be the greatest operational challenge we’ve ever faced as a nation, and we’re going to get it done,” he said.

While millions of doses have been administered across the country and the distribution process was still in a nascent stage, President Donald Trump‘s overpromising in an election year raised expectations that the government and pharmaceutical companies could not meet.

Dr. Anthony Fauci, the federal government’s top infectious disease expert, said in an interview with CNN Tuesday that “we are below where we want to be.”

“We certainly are not at the numbers that we wanted to be at the end of December,” Fauci, the director of the National Institute of Allergy and Infectious Diseases, said.

Trump said in October that “we’re on track to deliver at least 100 million doses of a vaccine this year.”

But in November, Pfizer – which makes one of two vaccines to have received emergency authorization from the federal government – drastically adjusted its predictions for production, and the administration began dropped the expectation to 40 million doses by the end of the year. Since both Moderna and Pfizer’s vaccines require two doses per person, that would be enough for 20 million Americans to get vaccinated.

“By Christmas, 20 million Americans by the end of this year, 20 million Americans could be vaccinated,” Health and Human Services Secretary Alex Azar declared earlier this month at a vaccine event.

But with just a few days left in 2020, it appears the United States will fall far short of even that.

As of Tuesday morning, only 11.5 million doses had been distributed and officials said they don’t expect to hit the 20 million mark until the first week of January.

Also, while Azar suggested that would be enough for 20 million people to be “vaccinated,” it’s not clear how many people will have actually received the shots. According to the Centers for Disease Control and Prevention, about 2.1 million people have been recorded as receiving a dose. But that number isn’t a good indication of real-time injections as it’s up to states and local authorities to report data and are given several days to do so.

“We know that’s underreported because there’s a three to seven-day delay. But we expect that to ramp up,” said Adm. Brett Giroir, a U.S. Department of Health and Human Services official, said in an interview with MSNBC on Tuesday.

Overall, the final numbers are a far cry of what Trump and his top aides promised early in the year, such as when Moncef Slaoui, the chief scientific adviser to the federal government’s vaccine program, said in May he was “confident that we will be able to deliver a few hundred million doses of vaccine by the end of 2020.”

Federal officials remained hopeful the pace would pick up.

Fauci said he thought “that as we get into January, we are going to see an increase in the momentum” that, he said, he hoped “allows us to catch up to the projected pace that we had spoken about a month or two ago.”

For months, the Trump administration has touted the support it has provided pharmaceutical companies with producing and distributing COVID-19 vaccines, helping lead to more than one receiving authorization for emergency use in record time.

But while its vaccine program, called “Operation Warp Speed,” has focused on getting millions of doses from the manufacturers to hospitals across the country, the federal government has largely left the final steps to overstretched state health departments. Experts have raised questions about whether it has done enough to help hospitals and pharmacies work through logistical challenges.

“The biggest problem is getting the vaccine from the states into people’s arms,” Dr. Ashish Jha, the dean of Brown University’s School of Public Health, said in an interview with ABC’s “Good Morning America” on Tuesday. “There’s a lot of steps and there hasn’t been much planning. There hasn’t been much investment.

“And we’re starting to see departments of health that are really stretched having to try to figure out how to get all these vaccines into people,” he added. “And it’s going much more slowly than I think the federal authorities thought it would.”

Giroir notes that all public health operates through state and local communities. For example, he said, the U.S. rolls out flu vaccines every year in much the same way.

It would be a dramatic departure if the federal government decided who gets vaccinations and where in the states. Administration officials say their role has been to provide money, supplies and support.

“The federal government doesn’t invade Texas or Montana and provide shots to people. We support the state and locals doing that,” Giroir told MSNBC.

Indeed, the federal government has left decisions about who actually can receive up to state and regional officials, with the CDC making non-binding recommendations. So far, though, healthcare workers have generally been prioritized across the country, in line with those guidelines.

The Trump administration’s approach to vaccine distribution to some degree has mirrored its approach to testing for COVID-19, which it also left largely up to the states to administer. As a result, a confused, haphazard testing regime across the country left massive gaps in public health officials’ understanding of the viruses’ spread – crippling the country’s response to the pandemic.

It was not immediately clear whether the vaccine rollout would follow a similar path with Giroir and other officials insisting that the vaccinations will ramp up dramatically in coming weeks.

Meanwhile, Vice President-elect Kamala Harris and her husband, Doug Emhoff, got the first dose of a COVID-19 vaccination on Tuesday morning. Biden received his first dose last week, while outgoing Vice President Mike Pence and his wife got theirs the week before.

ABC News’ Molly Nagle contributed to this report.