By Andy Sullivan

Oct 7 (Reuters) - Wednesday's matchup between Vice President Mike Pence and Democratic challenger Senator Kamala Harris is unlikely to bring the fireworks that marred last week's chaotic debate between President Donald Trump and former Vice President Joe Biden.

But Trump's COVID-19 infection, and his and Biden's advanced age, makes this perhaps the most consequential U.S. vice presidential debate in living memory. Here are a few things to watch when they get started at 9 p.m. EDT (0100 GMT Thursday):

OUTSIZED IMPORTANCE

In 2008, when he was Barack Obama's running mate, Biden said that "no one decides who they're going to vote for based on the vice president." Yet this debate has outsized importance.

Biden, 77, would be the oldest president in U.S. history if he were to win the election, and he has hinted he might only serve one term.

Trump, only slightly younger at 74, spent the weekend at a military hospital outside Washington after contracting the novel coronavirus.

From purely an actuarial standpoint, Pence, 61, and Harris, 55, would be more likely to step into the presidency than other vice presidential candidates. That is sure to be a major subtext on Wednesday night.

Pence also carries an additional burden as he has been tasked with campaigning for the ticket as Trump has been sidelined because of his COVID-19 infection.

BARRIERS AND MASKS

Eager for every advantage, campaigns often battle over everything from the format of the debate to the height of the podiums.

This time around, they have been at odds over coronavirus protections. The Salt Lake City debate stage will feature a Plexiglas barrier between the two candidates, which was installed over the objections of Pence's aides. They also will stand more than 12 feet (3.6 m) apart on stage.

Debate organizers handed out salmon-colored masks to all debate attendees and cordoned off alternative rows in the theater where the debate will be held to promote social distancing.

It will be a potent reminder that opinion polls show the coronavirus is the most important issue for voters, outstripping traditional concerns like the economy and national security. Pence has headed the Trump administration's coronavirus task force as the pandemic has killed more than 210,000 Americans and shut down wide swaths of the economy.

Members of Trump's entourage refused to wear masks at last week's debate in Cleveland, just two days before he and his wife, Melania, tested positive. The Commission on Presidential Debates has said anyone who does not wear a mask will be “escorted out.”