(CNN) With the hours dwindling until Joe Biden is sworn in -- officially taking the helm of the US government during its worst health crisis in 100 years -- a sense of nervousness has set in among those advising the incoming President on the pandemic.
The overarching, nagging concern: "They don't know what they don't know," said a source close to the Biden Covid-19 team.
Biden is set to inherit a nation grieving hundreds of thousands of Americans who have perished from the virus, a health care system buckling under the strain of the pandemic, new variants of the disease popping up around the world and a public that is both stressed about the prospect of when they can get vaccinated, as well as dubious about whether to trust the vaccine when it's their turn in line.
Multiple officials familiar with the transition said the lack of full cooperation and transparency from the outgoing Trump administration has contributed to Biden's Covid team feeling frustrated and concerned about having a full understanding of the scope of the problems they will confront on Day One.
But the President-elect's team feels ready for the fight.
"We're not going to hide from the fact that is going to be a tremendous effort that is going to require the hard work of millions of Americans," said Rep. Bill Foster, an Illinois Democrat on the House coronavirus subcommittee who has participated in briefings with Biden's transition team and was describing their posture coming into office. "It's not going to be some magical solution."
The President-elect understands just how much the success of his presidency will depend on his ability to get the virus under control, the source close to Biden's Covid operation said. The President-elect is anxious and focused on the pandemic right now, which looms large over Wednesday's inauguration.
Chief among the Biden team's concerns right now is vaccine supply and turning around the lackluster distribution effort, though new strains of the virus are another persistent worry for the incoming team.
One source familiar with the Biden effort acknowledged a major gap the incoming team will have to confront is that there is currently no effective and trustworthy line of communication between states and the federal government.
Ramping up federal involvement in coronavirus testing and vaccine distribution starts with building up communications with each state to better understand their infrastructure and supply challenges, something that's expected to be a focus in the early days of Biden's presidency.
States are clamoring for the federal government to release larger batches of vaccine. But it's still not clear if there will be enough vaccine available to drastically speed up the pace at which they can be distributed and administered.
While there is optimism that additional vaccines will be soon approved for use in the United States, including the potential of a single dose Johnson & Johnson vaccine, there are questions about whether it can be produced fast enough to significantly address any potential supply problems.
Foster, the Illinois congressman, said getting information from the Trump administration on delivery schedules and production progress for vaccines "has been like pulling teeth."
"And I understand the frustration of the Biden (team), if they were seeing the same sort of resistance we saw, on not what was contracted and promised but what the actual milestones in the vaccine production are along the way," he added.
Outgoing health officials in the Trump administration, meanwhile, insist they have been cooperative with the Biden team and have had hundreds of meetings with Biden's transition team.
Still, the Biden team likely won't get a full scope of the vaccine production landscape until he takes office Wednesday.
The President-elect's team has expressed concerns about trying to get a grasp on exactly how the vaccine distribution is playing out -- and what's slowing it down -- across all 50 states.
Even though Biden's team is fully aware they can't federalize the vaccine distribution process with the snap of a finger, the President-elect has vowed that the federal government will play a much more aggressive role in streamlining the vaccine distribution and Covid containment efforts.
But even when it comes to some signature promises -- like deploying the National Guard to run vaccination sites -- it will most likely fall to each state to determine what works best for them.
New variants of the coronavirus could prove to be another vexing problem for the incoming administration.
The worst-case scenario for Biden's team would a variant that cannot be treated by currently approved vaccines. But America's inferior screening systems for monitoring new strains of the virus, combined with the fraught relationship with the outgoing Trump administration, adds emerging variants to the thorny list of problems a Biden administration won't be able to fully tackle until he takes office.
Containing any new variant of the disease played into the Biden team's decision to immediately throw cold water on President Donald Trump's order lifting some travel restrictions on incoming travelers from much of Europe, the United Kingdom, Ireland and Brazil. While the outgoing President ordered those restrictions to be lifted on January 26 -- nearly a week after Biden takes office -- Biden's team said the President-elect would block the order once he's in office.
"With the pandemic worsening, and more contagious variants emerging around the world, this is not the time to be lifting restrictions on international travel," incoming White House press secretary Jen Psaki tweeted Monday. "In fact, we plan to strengthen public health measures around international travel in order to further mitigate the spread of COVID-19."