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

Trump reached out to Pennsylvania State Speaker of the House Brian Cutler last week asking about the legislature possibly overturning the election as part of his ongoing and apparent pressure campaign to have GOP-controlled legislatures flip results in his favor in battleground states he lost.

Mike Straub, a spokesman for Cutler, confirmed to ABC News reporting in The New York Times that Trump asked what options were available to the legislature on the phone call.

“Cutler made it very clear what power the legislature has and does not have," Straub told ABC News, adding he is "not aware of the President explicitly asking to turnover [sic] the election results."

Straub separately told ABC News that he was not present for the conversations between Cutler and the president, but he was briefed.

"The president wanted to know what options were available to the legislature to address those concerns... Speaker Cutler was very clear in explaining what power the legislature has and does not have within our state Constitution," Straub said. "One remedy the Trump campaign is seeking in a court case involves the legislature seating electors -- Speaker Cutler explained the legislature does not have that authority."

Cutler separately signed onto a resolution last week imploring Pennsylvania Republicans in Congress to officially contest the election results in January. While this has happened in the past, including over the election of George W. Bush, it is highly unlikely to overturn the election results.

The Supreme Court may decide Tuesday to take up a challenge by Pennsylvania Republican Rep. Mike Kelly over the constitutionality of mail-in voting, but legal experts have told ABC News that is a long shot. With Tuesday being the constitutionally mandated "safe-harbor" deadline for electors, all legal challenges are meant to be resolved by the end of the day, thus ensuring the correct electors will cast their votes in statehouses on Dec. 14.

-ABC News' Alex Hosenball