Democratic Majority Whip Jim Clyburn, whose primary endorsement in South Carolina helped put Biden on a path to victory and who was tapped this week to chair Biden’s presidential inaugural committee, began to paint a clearer picture Tuesday of what the public can expect on Jan. 20, saying a majority of the inauguration will be virtual.

"I think the president will be sworn in in a a traditional way, but 75%, 80% of this inauguration will probably be virtual," Clyburn told CNN Tuesday morning, comparing the look to this summer’s Democratic National Convention.

"This inauguration will be an example of what a President Joe Biden would like to see the people of America do," Clyburn added, noting the worsening coronavirus pandemic. "We are not going to violate anything... We are going to discourage anything that could be a spreader."

Providing an alternative, Clyburn said he's hopeful there could be a celebration on the National Mall July 4, adding, "Hopefully things will be under control then."

As Biden is expected to name retired Army Gen. Lloyd Austin to lead the Pentagon at some point this week, Clyburn also offered praise for the "historic" choice of and encouraged Biden to consider more Black candidates he's suggested -- including Rep. Marcia Fudge, D-Ohio, and Atlanta Mayor Keisha Lance Bottoms -- for other senior-level Cabinet positions. The pressure comes ahead of Biden's meeting with civil rights leaders Tuesday afternoon as they advocate for more people of color to be nominated to top spots.

"There are plenty of bodies to be found if you ask the right person," Clyburn said.

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

Shortly before releasing his press release confirming retired four-star Gen. Lloyd Austin as his pick to lead the Pentagon, The Atlantic magazine published an op-ed penned by Biden op-ed, laying out the reasons behind his choice and noting their history together in the Obama administration.

“Today, I ask Lloyd Austin to once more take on a mission for the United States of America—this time as the secretary-designate of the Department of Defense. I know he will do an outstanding job,” Biden wrote.

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. If confirmed, he would be the first African American to lead the Pentagon.

Biden pointed to Austin’s ”many strengths and his intimate knowledge of the Department of Defense and our government” as factors that made him “the person we need in this moment," saying his experience leading the Iraq drawdown prepares him for coordinating vaccine distribution and connecting with American families.

“And the next secretary of defense will have to make sure that our armed forces reflect and promote the full diversity of our nation. Austin will bring to the job not only his personal experience, but the stories of the countless young people he has mentored. If confirmed, he will ensure that every member of the armed forces is treated with dignity and respect, including Black, Latino, Asian American, Native American, women, and LGBTQ service members," Biden wrote.

Biden also seemed to address the fact that Lloyd’s nomination would require a waiver given his recent military service -- something that some Democrats have already expressed opposition to.

“I respect and believe in the importance of civilian control of our military and in the importance of a strong civil-military working relationship at DoD—as does Austin," he wrote.

"Austin also knows that the secretary of defense has a different set of responsibilities than a general officer and that the civil-military dynamic has been under great stress these past four years," Biden added.

The announcement comes as Biden and Vice President-elect Kamala Harris meet with civil rights leaders who have pushed Biden to name more people of color to senior-level Cabinet positions.

-ABC News' Molly Nagle