Image source, Getty Images

The second topic of the night is readiness to lead - could the candidates take over as commander in chief at a moment's notice? A few polls released this week across the US asked voters to weigh in on which running mate would be best suited to take over the presidency in a worst-case scenario.

A Wednesday poll, external by the St Leo University Polling Institute found that just over 48% of national likely voters would prefer a President Harris to a President Pence (though her lead dropped significantly in Florida, where the institute is based).

Earlier this week, a Tuesday poll by The Boston Herald and Franklin Pierce University found 45% of voters preferred Harris and 40% preferred Pence.

A YouGov poll, external found that while voters said Harris was a strong leader, among registered voters, Pence had the edge when asked about qualifications to step-in as president, with 50% saying yes to 43% for Harris.

Harris has a quick retort: the man standing next to her on stage was, in fact, the head of the task force. And yet, people are struggling.

Pence says the administration has put the American people first throughout the crisis.

Page now asks the vice-president about an outbreak centred at the White House. How can the nation follow the White House guidelines when politicians there haven't done so?

Pence says Americans have been given the information and are putting it into practice.

"President Trump and I trust the American people to make choices in the best interest of their health," Pence says, adding that they respect Americans' choices.

The last question in the Covid section is about vaccines. Would the candidates take it the vaccine?

Harris says if the experts, like Dr Anthony Fauci, say they should take it, she'll be first in line.

"But if Donald Trump tells us we should take it? I'm not taking it."

Reality Check

Kamala Harris said the president called coronavirus a “hoax”, but this isn’t right.

In February, President Trump accused Democrats of "politicising the coronavirus" then mentioned the impeachment process against him, calling that a "hoax" and criticism of his handling of Covid-19 "their new hoax."

He didn't directly call the virus a "hoax".

Pence is touting Trump's leadership in this crisis. Trump, he says, “did what no other president had ever done - suspended all travel from China, the second-largest economy in the world.” The vice-president claims this saved numerous lives.

He echoes Trump's claim that there will soon be "tens of millions of doses" of the vaccine available, and dismisses Biden's Covid-19 plan as "plagiarism" - a reference to a long-ago scandal involving the Democrat plagiarising a speech by British Labour leader Neil Kinnock.

Image source, Reuters

Anthony Zurcher
BBC North America reporter

Not surprisingly, the coronavirus pandemic was the topic of the first question at the vice-presidential debate – and not surprisingly, Kamala Harris spent most of her two-minute answer on the attack and Mike Pence spent most of his on defence. Such is the reality of the political situation today, given that it’s the Trump administration currently in charge.

Harris said what the administration is doing isn’t working, but Pence’s has thought through his defence - to say that is any criticism of his administration is an attack on first-responders and US healthcare workers.

Moderator Susan Page asks why death toll as a percentage of the US population is higher than nearly any nation on earth - and twice that of neighbouring Canada.

"From the very first day Donald Trump has put the health of Americans first," says Pence.

Trump, he says, "did what no other president had ever done - suspended all travel from China, the second largest economy in the world."

He says that Trump's decision saved hundreds of thousands of lives.

The first question for the VP debate concerns coronavirus - a subject that looms over the whole campaign and the country.

Support for Donald Trump’s handling of the pandemic fell to its lowest point among Republicans in July, according to data from Ipsos, a leading polling company - but it has recovered a little since then. The last time a majority of Americans approved of his handling was back in April, the data suggests.

Read more: Trump says not to fear Covid-19. Do Americans agree?

Directly addressing the audience, Senator Harris accuses the Trump administration of knowing what would happen: "They knew and they covered it up."

"They minimised the seriousness of it. The president said 'you're on one side of the ledger if you wear a mask, you're on the other side if you don't'".

She adds: "They still don't have a plan, well Joe Biden does."

That plan is focused on contact tracing and vaccines.

She says the Trump administration has "forfeited" the right to re-election because of their response to Covid-19.

It's the defining issue of 2020 and no surprise that the coronavirus pandemic is the subject of the first question to the vice-presidential candidates. Mike Pence is in charge of the presidential task force on the pandemic, so he's going to be under pressure to defend the Trump administration’s handling of the 7-month-old health crisis that has killed more than 210,000 Americans.

Harris has to walk a tightrope here. Pundits think she'll avoid attacking the president personally, while he recovers from Covid-19, but she'll undoubtedly continue to pan the White House response. We'll see where she goes now, but voters will want to see solutions and plans for improvement, not just criticism.

"The coronavirus is not under control," Page begins.

"What would a Biden administration do in January and February that a Trump administration wouldn't do?" she asks Harris.

Read more: Tracking the global coronavirus pandemic

Read more: Is the US the worst-hit country for deaths

Here we go, the first and only vice-presidential debate has officially begun at the Kingsbury Hall in Salt Lake City, Utah.

We've got veteran journalist Susan Page, the USA Today Washington bureau chief, in the moderator's seat. Vice-President Mike Pence and Senator Kamala Harris are ready to go at their podiums, and families are in the audience.

Page begins by noting that there are extra Covid precautions tonight following lessons-learned from last week's Trump-Biden debate.

Everyone in the audience must wear a face mask for one. And Harris and Pence are 12ft (3m) apart, instead of six.

She also notes that the campaigns have agreed to tonight's ground rules - and she's here to enforce them "on behalf of the millions of Americans watching".

Sophie Long
BBC News, Salt Lake City

With Donald Trump aged 74 and Joe Biden aged 77, there has been a lot of focus on the vice-presidential candidates and their fitness to lead if the matter ever arose.

Only last week, people were thinking: Is Mike Pence going to have to take over?

They are going to be looking at Kamala Harris and thinking, is she ready, could she take over as Commander-in-Chief at a moment's notice?

And Mike Pence is no less of a curiosity: We haven't seen him in a live TV debate for four years - since 2016.

The debate is just starting - here's what our voter panel is looking forward to seeing tonight.

Jim Sullivan, Trump voter: Same as everyone I'm looking for a substantive discussion of the issues. From my perspective, Mike Pence needs to make a case for why they should be in office for another term. Kamala Harris needs to come out and talk about her time as a defence attorney. I'm looking for a lot less drama [than at the presidential debate] and looking for a lot more civil dialogue where issues and policy positions are covered in more clear detail.

Akayla Sellers, Biden voter: During these times of racial uproar I'm looking forward to seeing Senator Harris confront her controversial past, which we know Vice-President Pence will use against her. I'm also looking forward to seeing the juxtaposition to her upcoming policing reform policies.

Shloka Ananthanarayanan, Biden voter: A more substantive and coherent discussion of the issues that matter to the American public and a clear articulation of how the parties differ in their approach to issues like healthcare, economic policy and voting rights. As a woman, I am particularly keen on seeing if they will broach Roe v Wade and reproductive rights, given that Pence and Harris espouse such differing viewpoints and we currently have a battle for the Supreme Court that could have lasting consequences for this particular issue.

Gordon Kou, undecided voter: I'm looking forward to seeing how the VP candidates hold their own in defending their party's nominee and platform. VP candidates are also early frontrunners for future elections, so I think it is important to know who they are and where they stand.

Read more about our voters.

We're now just five minutes out from the only vice-presidential debate of 2020.

To watch the live event on BBC News, click on the link at the top of this page.

Image source, bbc

Amanda Renteria, who was national political director for Hillary Clinton during her 2016 presidential run, tells BBC News we're about to see a historic moment as Kamala Harris takes the stage.

"There's a lot of women, a lot of women of colour [who are] seeing themselves in her," she says.

"Tonight is to give some credibility as to what it would look like, should she have to step in as president."

[...] "This is when leaders rise."

There have only been three female vice-presidential candidates, and one woman presidential candidate - all of whom were white.

If Kamala Harris has waited all her life for this moment, says Renteria, a lot of others have waited as long to see it.

Image source, Getty Images

Ritu Prasad
BBC News writer, Fort Myers, Florida

We’ve been asking our readers for their most pressing questions about the US election. Now it’s our turn to respond.

Click here if you want to know more about this project or send in a question of your own.

Sam C, 51, from Auckland, New Zealand, asks: If Joe Biden were to become incapacitated or die from Covid-19 before the election, and Kamala Harris became the nominee, would she require a running mate? What would happen given that ballot papers have already been printed?

To answer the last part of your question first: it’s far too late in the election season to make real ballot changes. Millions of voters have already posted or dropped off ballots (here in Florida, we’ve already received ours), or visited early polling sites.

Should Biden become unfit to serve before the election, it’s not set in stone that Harris would become the new presidential nominee. That’s up to the Democratic National Committee, who could name a replacement.

When US voters go to the polls, though they circle one candidate, they’re actually voting for “electors”, who are party appointees that will vote in their stead.

If a nominee dies, these electors could instead cast their vote for whoever the party picks (though some states have laws binding how electors vote that would complicate this).

If Biden dies or can't serve after winning the election, then Harris automatically becomes the president.

Facebook has announced a new series of measures regarding the US election.

Noting that final election results may take longer to come in than usual due to the pandemic and postal voting, the social media giant said on Wednesday it was trying to "prevent the spread of misinformation".

New measures include:

- Notifications on posts saying the count is ongoing if a "candidate or party declares premature victory before a race is called by major media outlets".

- Banning content that "seeks to intimidate voters", including "calls for people to engage in poll watching when those calls use militarised language or suggest that the goal is to intimidate".

- Not running political or social issues adverts after the polls close on Election Day

Facebook recently banned adverts seeking to "delegitimise any lawful method or process of voting".

Read more: Social media's nightmare scenario

Image source, Getty Images

Senator Kamala Harris is a history-making vice-presidential pick already by being the first black, Indian woman nominated to the position.

If the Biden-Harris team wins in November, she'll make history once more as the first woman to be a heartbeat away from the most powerful office in the world.

So as we wait for this historic vice-presidential debate to take off, let's take a look back at the other women who helped ready the stage for a candidate like Kamala Harris.

Read the full story: Women who paved way for first black female VP hopeful

The internet never forgets - and as tonight's debate approaches, some are recalling an earlier clash between Mike Pence and a female foe - Disney's Mulan.

When the original animated film was released in 1998, Pence, then a conservative radio host in Indiana, saw stealthy propaganda.

In an op-ed, he called the film an attempt by some "mischievous liberal" at Disney to influence the debate over women in the military.

"Despite her delicate features and voice, Disney expects us to believe that Mulan's ingenuity and courage were enough to carry her to military success on an equal basis with her cloddish cohorts," Pence wrote.

Unfortunately, his claims about "liberal propaganda" betrayed Pence's ignorance. The female warrior Hua Mulan is perhaps one of the most famous historical figures in China, and the basis for the film.

Image source, EPA

The US Marine Corp's number two officer has tested positive for the coronavirus, the service has announced. It comes amid an outbreak among the highest echelons of the US military and among White House officials.

Assistant Marine Commandant General Gary Thomas is experiencing mild symptoms, a statement said. All but one of the joint chiefs of staff - who oversee America's five military branches - are in isolation after exposure to an infected service member.

Read more: Aide tests positive as military leaders quarantine

The University of Utah, which is hosting tonight's debate, is putting out official bingo cards, external that people can download to play along from home. The cards include hot button terms such as "mental fitness", "systemic racism", "masks" and "Dr Anthony Fauci" - the top US disease expert who has become household name amid the coronavirus pandemic.

"Blue states", "Black Lives Matter" and "Vladimir Putin" also are in play. There are 10 different unique cards put out by the university.

People on Twitter are also playing their own version of bingo, but many of their word choices are... a bit less wholesome.

Image source, University of Utah