Trump and Biden hold dueling events on the pandemic response Tuesday with the president hosting a "vaccine summit" at the White House, just days before a Food and Drug Administration advisory committee holds a hearing on whether to authorize emergency use of a COVID-19 vaccine, while the president-elect is slated to formally roll out his health care picks who will take over handling the COVID-19 pandemic in 43 days.

At his event, Trump is slated to sign an executive order touting his mantra of "America First" that would prioritize Americans’ access to COVID-19 vaccines before the United States helps other countries. But the chief science adviser to "Operation Warp Speed," the U.S. government's initiative to expedite vaccine development which Trump is also expected to tout, said Tuesday morning he doesn't know about the vaccine-related executive order Trump is expected to sign.

"Frankly, I don't know and, frankly, I'm staying out of this. I can't comment," Dr. Moncef Slaoui told ABC News Chief Anchor George Stephanopoulos in an interview Tuesday on "Good Morning America,” when asked to explain the executive order.

The event comes as a senior administration official familiar with the matter confirmed reporting to ABC News that the Trump Administration passed when Pfizer offered in later summer to sell the U.S. more COVID-19 vaccine doses -- an approach which the White House has denied and Slaoui defended Tuesday. Representatives of Pfizer and Moderna -- the two drug companies seeking emergency-use authorization -- are not expected to attend the White House summit, nor is anyone from the Biden transition team.

Biden, meanwhile, is slated to introduce the health care team he will entrust with handling the pandemic California Attorney General Xavier Becerra as Health and Human Services secretary, the first Latino to hold the post if confirmed. Director of the National Institute of Allergy and Infectious Diseases Dr. Anthony Fauci, who is taking on the elevated role of chief medical adviser to Biden in the incoming administration, is expected to dial into Biden’s meeting, though not the one at the White House.

Biden and Vice President-elect Kamala Harris are also meeting Tuesday with several civil rights leaders including NAACP representatives to discuss policy issues around racial equity as well as diversity and representation on the incoming Cabinet. It comes after news leaked that Biden is expected to name retired four-star Army Gen. Lloyd Austin as his defense secretary, who would be the first African American in the post if confirmed, this week.

Tuesday is also the congressionally mandated "safe harbor" deadline -- the date, under federal law, by which the "final determination of any controversy or contest concerning the appointment" of electors "shall be conclusive” -- making it extremely difficult to dispute election results in the courts, where pro-Trump efforts have seen at least 39 defeats to date.

President-elect Joe Biden is expected to name retired four-star Army Gen. Lloyd Austin as his choice to lead the Defense Department, four sources familiar with Biden's plans told ABC News.

If confirmed, Austin would be the first Black defense secretary in American history.

Austin, the former commander of U.S. Central Command -- with jurisdiction over military activities in the Middle East -- retired in 2016 after more than 40 years of military service, including a stint leading U.S. forces in Iraq and the campaign against the Islamic State.

Austin will need a waiver from Congress to lead the Pentagon, given that he is less than seven years from active duty. Congress granted retired Marine Corps Gen. Jim Mattis the same waiver in 2017 to allow him to serve as President Donald Trump's first defense secretary.

-ABC News Chief Global Affairs Correspondent Martha Raddatz, Benjamin Siegel, Katherine Faulders, John Santucci and Luis Martinez

Biden will announce other members of his economic and domestic Cabinet this week, in addition to his pick for defense secretary, according to a statement from the office of the president-elect.

"President-elect Biden will announce additional members of his cabinet before Christmas, including his nominee for Secretary of Defense and members of his economic and domestic cabinet before the end of this week,” the statement reads.

While Biden was asked about his attorney general pick earlier Monday -- along with secretary of defense -- the campaign notably does not specify that that position will be rolled out this week in the new statement.

-ABC News' Molly Nagle

Biden confirmed more Cabinet picks are expected to come this week, saying he would announce his Defense Secretary on Friday.

While Biden rolled out a portion of his national security team as part of his first Cabinet announcement, there has been continued speculation over his choice to lead the Justice and Defense Departments. Biden in recent days has been urged to appoint a person of color, particularly a Black person at the Department of Defense and as the Attorney General.

The speculation comes ahead of Biden meeting with the NAACP on Tuesday.

-ABC News' Molly Nagle

With just hours to go until Tuesday's "safe harbor" federal law-mandated deadline for states to certify their election results and next Monday's Electoral College voting coming up, time is running out for Trump to find a way to overturn the results of the presidential election as he’s set out to do.

The president held a rally in Georgia -- his first since losing the election -- to campaign for the upcoming Senate runoffs there, races which will determine whether the GOP maintains control, but instead he spent most of his 101 minutes on stage pushing a whirlwind of unsubstantiated claims that the election was stolen.

Trump directly called on GOP Gov. Brian Kemp to "immediately ask for a special session of the legislature" -- which could theoretically set up the process of allowing lawmakers to appoint new electors who would support Trump over Biden despite Biden winning by nearly 12,000 votes in the state.

But Georgia officials have rejected Trump's call.

Kemp, joined by GOP Lt. Gov. Geoff Duncan, said in a statement late Sunday it would be unconstitutional for the legislature to "retroactively change" the process for choosing presidential electors, saying "doing this in order to select a separate slate of presidential electors is not an option that is allowed under state or federal law."

While Kemp and Duncan, along with GOP Secretary of State Brad Raffensperger and officials in his office, have said the only option in disputing the results is through the courts, pro-Trump efforts have ended in at least 38 defeats to date, with only a single court victory -- in a case in Pennsylvania that was ultimately not consequential.

It all comes as the president’s personal attorney tasked with leading his efforts to overturn the election in court, Rudy Giuliani, who has been traveling the country as part of the president's ongoing efforts to dispute election results, remains hospitalized with COVID-19 at MedStar Georgetown University Hospital in Washington since Sunday. The Arizona legislature has closed for the week "out of an abundance of caution" after Giuliani met with roughly a dozen GOP lawmakers.

As deadlines loom, Trump on Monday presents the presidential Medal of Freedom to wrestling legend Dan Gable at noon and meets with Vice President Mike Pence behind closed doors.

And while Trump administration officials vow that with vaccine on the horizon, life will soon return to normal, the Biden team is sounding alarms on its distribution, saying they still don’t have access to the plan to roll it out.

"We have yet to see any kind of detailed plan," Dr. Celine Gounder, an expert on Biden’s COVID-19 advisory board, told "CBS This Morning" Monday after Biden said the same last week.

Pressing forward with the transition despite Trump's roadblocks, Biden and Vice President-elect Harris will receive the President’s Daily Brief and meet with transition advisors on Monday ahead of personally debuting their new health picks, who will inherit leading the country through the ongoing coronavirus pandemic.