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

Up against Tuesday's "safe harbor" deadline, Georgia Secretary of State Brad Raffensperger said that he will recertify that Biden won Georgia's election sometime Monday following the state's third recount of the presidential vote there which has Biden ahead by nearly 12,000 votes.

"It's been a long 34 days since the election on Nov. 3. We have now counted legally cast ballots, three times, and the results remain unchanged," Raffensperger told reporters.

As he did in his WSJ op-ed this morning, the secretary equated how former gubernatorial candidate Stacey Abrams acted after her 2018 loss and Trump's behavior in 2020.

"Whether it's the president of the United States or a failed gubernatorial candidate ... disinformation regarding election administration should be condemned and rejected," he said. "All this talk of a stolen election, whether it's Stacey Abrams, or the president of the United States is hurting our state."

Raffensperger also said that "the focus on Nov. 3 is drawing energy away from" the state's goals of job growth, efficient COVID-19 vaccine distribution and "getting back to normal."

"I know there are people that are convinced the election was fraught with problems, but the evidence -- the actual evidence, the facts -- tell us a different story," he said.

Gabriel Sterling, the voting system implementation manager in Raffensperger's office, came to the podium afterward to fact check some of the misinformation that the president has helped spread. In one example, he directly called out the president's legal team for how they've tried to "mislead" people about a video from counting occurring in State Farm Arena in Fulton County that Trump played at his rally in Valdosta on Saturday night.

"What's really frustrating is the president's attorneys had this same videotape. They saw the exact same things the rest of us could see, and they chose to mislead state senators and the public about what was on that video," he said, debunking the notion that there were "magic ballots" that showed up in the state's largest county.

"They knew it was untrue and they continue to do things like this," Sterling said. "We continue to see people who are put in positions of responsibility, sending out this disinformation and undermining the electoral system," he added later.Minutes after Raffensperger told reporters he would receritfy the vote Monday, Trump continued his attacks on election officials in the state on Twitter, targeting GOP Gov. Brian Kemp for signature verification saying he'd have an "easy win" were it conducted -- but signature verification was already done twice for absentee ballots in the state, and the vote was recounted three times affirming Biden's win.

-ABC News' Quinn Scanlan