Former Vice President Joe Biden on Sunday night made a firm commitment to picking a woman as his running mate during his first one-on-one Democratic primary debate with progressive Senator Bernie Sanders.

"If I'm elected president, my Cabinet, my administration, will look like the country and I commit that I will in fact pick a woman to be my vice president," Biden said during the debate.

"There are a number of women qualified to be president tomorrow and I would pick a woman to be my vice president," said Biden.

Sanders said he was almost certain to pick a woman as his running mate if he can win the nomination.

"In all likelihood, I would," Sanders said, adding that it's more important for him to choose a progressive to run alongside him.

"There are progressive women out there," said the senator.

The face-off came amid worries about the rapid spread of coronavirus across the United States. There was no handshake between them and no audience in the room. The two candidates stood six feet (1.8 meters) apart at the podiums.

The pandemic became the central topic during the debate. Sanders used the outbreak as an opportunity to push his signature promise "Medicare for All", while Biden argued that Americans are "looking for results, not a revolution" and that it was not the time for massive structural change to the U.S. economy.

The United States first has to address the ongoing public health crisis, said Biden.

On Thursday, one day after Trump's national address, Biden called for making virus testing free and widely available, setting up 10 mobile sites and drive-thru facilities per state, as well as more transparency from the White House on testing, according to a report from The Hill website.

Biden currently leads Sanders in the Democratic race.