Openly contradicting top US health experts, Donald Trump has predicted a safe and effective coronavirus vaccine could be ready as early as next month.
Undermining the director of the Centers for Disease Control and Prevention and calling him "confused" in projecting a longer time frame, the president said on Wednesday a mass distribution would follow soon after.
Trump also disagreed with Dr Robert Redfield about the effectiveness of protective masks and said he'd telephoned him to tell him so.
Earlier, the CDC sent all 50 states a "playbook" for free distribution of a vaccine to all Americanswhen one is proven safe and effective.
Redfield told a congressional hearing health care workers, first responders and others at high risk would get the vaccine first, perhaps in January or even late this year.
But it was unlikely to be available more broadly, again assuming approval, before late spring or summer.
After Trump's comments, CDC officials said the director had thought he was answering a question about when vaccination of all Americans might be completed.
Redfield told the Senate hearing of the importance of everyone wearing protective masks to stop the pandemic, which has killed nearly 200,000 Americans.
He floated the possibility of a vaccine being 70 per cent effective in inducing immunity, and said, "I might even go so far as to say that this face mask is more guaranteed to protect me against COVID than when I take a COVID vaccine".
Trump would have none of that.
"Vaccine is much more effective than the mask," he declared.
As for vaccinating Americans, he said: "We think we can start sometime in October."
One of his recently added advisers, Dr Scott Atlas, said as many as 700 million doses could be available by the end of March.
Trump made the prediction even though the vaccine is still being tested in humans and some experts have said they believe a safe and highly effective vaccine is at least several months way.
Redfield said states are not ready to deal with distribution demands and some $US6 billion ($A8.2 billion) in new funding would be needed to get the nation prepared.
Unswayed, Trump said, "we're ready to move and I think it will be full distribution".
Redfield said any vaccine available in November or December would be in "very limited supply" and reserved for first responders and people most vulnerable.
The shot wouldn't be broadly available until the spring or summer 2021, he estimated.
Only about half of Americans said they'd get vaccinated in an Associated Press-NORC poll taken in May.
Since then, questions have only mounted about whether the government is trying to rush treatments and vaccines to help Trump's re-election chances.
As for the planned vaccine campaign, Redfield said his agency will work with state health officials to implement the preparations:
- For most vaccines, people will need two doses, 21 to 28 days apart.
- Initially there may be a limited supply and the focus will be on protecting health workers, other essential employees and people in vulnerable groups.
- The vaccine itself will be free.
- States and local communities will need to devise precise plans for receiving and locally distributing vaccines.
- A massive information technology effort will be needed to track who is getting which vaccines and when.