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

While the U.S. Supreme Court has twice refused to hear pro-Trump challenges to the 2020 elections, a federal judge in Wisconsin on Saturday joined the chorus of rulings against Trump in his effort to use the courts to invalidate Biden’s victory.

“This Court has allowed plaintiff the chance to make his case and he has lost on the merits,” wrote U.S. District Court Judge Brett H. Ludwig, a Trump appointee. Ludwig noted that the president had asked “that the Rule of Law be followed,” and he declared in response: “It has been.”

The ruling comes just one day after the U.S. Supreme Court refused to consider an election challenge brought by the Texas attorney general contesting the way elections were run in four states, including Wisconsin. Trump had called that case “the big one,” because he thought it held the best hopes for him of re-litigating the 2020 contest in court.

This latest ruling marks nearly 50 losses for the president  in cases brought by him and his supporters since election day. In Wisconsin, where Biden won by more than 20,000 votes, Trump asked for 221,000 absentee and mail-in ballots to be excluded on the grounds they were collected in ways not laid out by the state legislature. And the president argued that the legislature should be afforded the chance to select an alternate slate of electors.

Ludwig’s 23-page opinion gave wide latitude to Trump -- finding that the president had standing to file his election challenge and was not too late to raise his concerns about the way the election was conducted. But the outcome of the case was the same as rulings in other battleground states -- that Biden’s victory was attained legally and should not be thrown to a legislature to upend.

The president, Ludwig wrote, “has not proved” that state election officials violated his rights. “To the contrary, the record shows Wisconsin’s Presidential Electors are being determined in the very manner directed by the Legislature, as required by Article II, Section 1 of the Constitution.”

Ludwig further noted that if he followed the demands set out in Trump’s lawsuit, “any disappointed loser in a Presidential election, able to hire a team of clever lawyers, could flag claimed deviations from the election rules and cast doubt on the election results. This would risk turning every Presidential election into a federal court lawsuit over the Electors Clause.”

The Trump campaign has not yet responded to requests for comment.

At the moment the federal ruling was handed down, the Wisconsin Supreme Court was hearing arguments on a separate challenge to a recount of votes in the state, which had failed in a lower court.

-ABC News' Matthew Mosk and Alex Hosenball