WASHINGTON (AFP) - US President Donald Trump said on Friday (Sept 18) he expects enough Covid-19 vaccine "for every American" will be produced by next April, and that the first doses will be distributed immediately after approval later this year.

Claiming "historic progress" with three vaccines in the final stages of development and trials, Trump said at least 100 million doses would be manufactured by the end of the year - perhaps more.

"Hundreds of millions of doses will be available every month, and we expect to have enough vaccines for every American by April," he said.

Trump also vowed to get the vaccine out with lightning speed, saying, "I think distribution will go even quicker than most people think."

Vaccines are seen as one of the top methods for halting the spread of the coronavirus, which has killed nearly 950,000 people worldwide, including a global high of 198,000 in the United States.

The president, facing a tough reelection campaign, has repeatedly stated that he envisions a safe and effective vaccine approved in the coming weeks, even ahead of the Nov 3 presidential election.

But that extraordinarily optimistic timeline was called into question by a top government health official this week.

Centres for Disease Control and Prevention director Robert Redfield testified to lawmakers on Wednesday that a "very limited" distribution to priority groups, including first responders, could begin in November and December, but that full implementation would take many more months at least.

"I think we're probably looking at late second quarter, third quarter 2021" before a safe and effective vaccine would be broadly available to the general public, he added.

Trump's Democratic election rival, Joe Biden, also expressed doubt about the timeline, and told voters on Thursday in Pennsylvania that he does not trust Trump to put forward a vaccine absent of political interference.

"I don't trust the president on vaccines," Biden said, explaining that he trusts the government's top infectious disease expert Anthony Fauci instead.

"If Fauci says the vaccine is safe, I take the vaccine."