Trump says coronavirus vaccine could be weeks away

US President Donald Trump said a vaccine against coronavirus could be three or four weeks away, despite cautionary notes sounded by public health officials about accelerated timeline.

Trump, speaking at a town hall hosted by ABC News in Philadelphia, defended his handling of the coronavirus crisis, and said a vaccine could be ready for distribution before the US presidential election on November 3.

"We're very close to having a vaccine," he said. "If you want to know the truth, the previous administration would have taken perhaps years to have a vaccine because of the FDA and all the approvals. And we're within weeks of getting it... Could be three weeks, four weeks."

Earlier this month, top US infectious disease expert Dr. Anthony Fauci told CNN that most experts believe a vaccine will be ready by November or December. "It is conceivable that you can have it by October, though I don't think that that's likely." Other experts say a scientifically credible vaccine will not be available until early 2021.

Trump bristled at tough questions from uncommitted voters and ABC News host George Stephanopoulos during the town hall meeting, arguing that his decision to impose travel bans on China and Europe had saved thousands, if not millions, of lives.

He also defended Americans who have shunned face masks and social distancing guidelines, and said even experts like Fauci had changed their views about those practices during the crisis.

Trump has faced criticism for holding large-scale campaign events in Nevada and other states - events that his adviser Fauci has described as "absolutely" risky.

His Democratic challenger, former vice president Joe Biden, last week accused Trump of "dereliction" of duty in dealing with the pandemic, which has cost millions of jobs.

The United States has reported nearly 6.6 million cases of COVID-19, the disease caused by the virus, the highest number worldwide, and nearly 195,000 deaths. That accounts for 20% of the cases worldwide, although the United States has just 4% of the world's population.

Trump said the United States had a number of cases because it did more testing than other countries.

He also repeated his claim from early in the pandemic that the virus would disappear on its own, and denied understating the threat of the disease when asked by an audience member why he would "downplay a pandemic that is known to disproportionately harm low-income families and minority communities".

Trump also provoked mockery on Twitter when he spoke about "herd mentality" instead of "herd immunity," a form of indirect protection from infectious disease that occurs when enough people have become immune through vaccination or previous infections.

"It would go away without the vaccine... but it's going to go away a lot faster with it," he said. "You'll develop ... a herd mentality."