A COVID-19 vaccine could be ready by early December for frontline healthcare workers to receive in early 2021.
US Medical Chief Dr Anthony Fauci told the BBC a potential vaccine was not far away.
"We will know whether a vaccine is safe and effective by the end of November, the beginning of December," Dr Fauci said on the Andrew Marr Show on Sunday.
Fauci was asked if Donald Trump's claims that a COVID-19 vaccine would be coming by the end of the year were true.
"That could start by the end of this year, the beginning of January, February, March of next year," Fauci replied.
"The amount of doses that will be available in December will not certainly be enough to vaccinate everybody. You'll have to wait several months into 2021."
Healthcare workers and those with underlying health issues are likely to be given priority access to any vaccine.
The remarks come as the NHS in the UK announced that frontline NHS staff are set to receive a vaccine within weeks.
The NHS is preparing for a national vaccination programme before Christmas, an email from NHS Trust chief to staff, reported by the Daily Mail, revealed on Sunday.