Most health experts, including Robert Redfield, the director of the Centres for Disease Control and Prevention, have said a vaccine will not be widely available until mid-2021.

Trump: coronavirus vaccine weeks away

Trump has accused his rival of spreading “anti-vaccine rhetoric,” while Biden has emphasised that he will listen to scientists, not the president, regarding a vaccine’s safety.

The cable network described the event as a “drive-in town hall”: participants remained at their parked cars outside a minor-league baseball stadium to ensure they stayed safely distanced from one another.

04:37

Barack Obama and Joe Biden join forces in new campaign video targeting US President Trump

Barack Obama and Joe Biden join forces in new campaign video targeting US President Trump

“He knew it – he knew it and did nothing,” Biden said. “It’s close to criminal.”

Trump has subsequently said he was trying to avoid unnecessary panic.

Biden said he did not have the power to enforce a national mask mandate, walking back remarks he made on Wednesday in which he suggested the president could legally require masks amid a national emergency. But he vowed to encourage every governor to do so while criticising Trump for questioning the efficacy of masks.

Trump and Biden vow to reduce China dependency, but methods ‘night and day’

Meanwhile, an ex-adviser to vice-president Mike Pence has joined the chorus of former Trump administration officials urging voters to deny him a second term.

She said in a new video released on Thursday by the group Republican Voters Against Trump that the president was more concerned about his re-election chances than the virus. She said if Trump had taken the virus seriously, he would have saved lives.

Troye also claimed Trump once suggested the coronavirus might be a good thing because it would stop him from having to shake hands with “disgusting people.” She said she is a lifelong Republican who plans to vote for Biden for president.

The White House adamantly denied Troye’s allegations. Pence said her remarks read “to me like one more disgruntled employee that has decided to play politics during election year.”

Biden also took advantage of the event’s setting near his birthplace of Scranton, comparing his upbringing in a working-class town with what he derided as Trump’s “Park Avenue” background.

“Guys like me who were the first of my family to go to college … We are as good as anybody,” he said. “And guys like Trump, who inherited everything and squandered what they inherited, are the people that I’ve always had a problem with – not the people who are busting their neck.”

He said he would accept the results of November’s election, a stance that Trump has declined to adopt amid his unfounded claims that the widespread use of mail-in ballots due to the pandemic will cause massive fraud.

Electoral experts have said voter fraud is very rare in the US, where voting by mail is a long-standing practice in many states.

Asian-Americans prefer Biden to Trump

For much of the summer, Biden held mostly virtual events from his home in Delaware, drawing criticism from Trump that he was “hiding”. But Biden maintained an advantage over Trump in national opinion polls throughout, as the pandemic battered Trump’s standing among voters.

In recent weeks, Biden has begun to hold campaign events again in other states, but they have been largely devoid of attendees aside from reporters and invited guests in a nod to the coronavirus.

Trump, who has returned to holding large-scale rallies, took part in an ABC town hall with undecided voters earlier this week, where he defended his administration’s response to the outbreak.

At an outdoor rally in Mosinee, Wisconsin, on Thursday night, Trump told supporters that Biden “would absolutely eradicate your state’s economy” and mocked the CNN event for keeping attendees apart in cars.

Covid-19 has killed more than 195,000 people in the United States, the most of any country.

Additional reporting by Associated Press