It will be known before the end of the year whether or not there is a safe and effective coronavirus vaccine, the United States' top infectious disease specialist has said.
Dr Anthony Fauci predicted such knowledge could arrive by the end of November or beginning of December.
However, he warned there would not be enough doses to vaccinate everyone before 2021.
His comments come amid reports plans are being made for frontline health staff in the UK to receive a Covid-19 vaccine within weeks.
Appearing on the BBC's Andrew Marr Show, Dr Fauci was asked if US President Donald Trump's claims that a Covid-19 vaccine, while not a guarantee, will be coming by the end of the year were true.
Dr Fauci replied: "We will know whether a vaccine is safe and effective by the end of November, the beginning of December.
"The question is, once you have a safe and effective vaccine, or more than one, how can you get it to the people who need it as quickly as possible?
"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."
He said healthcare workers will likely be prioritised first for any vaccine, as well as people considered at increased risk of complications.
"We will know whether a vaccine is safe and effective by the end of November, beginning of December"— BBC News (UK) (@BBCNews) October 25, 2020
US government scientist Dr Anthony Fauci tells #Marr once a Covid-19 vaccine is available, the challenge will be distributing it to those who need ithttps://t.co/KgSEipzrUB pic.twitter.com/kkay0Y6mQm
Dr Fauci added: "That could start by the end of this year, the beginning of January, February, March of next year.
"When you talk about vaccinating a substantial proportion of the population, so that you can have a significant impact on the dynamics of the outbreak, that very likely will not be in to the second or third quarter."
Dr Fauci expressed concern that "anti-science" sentiment could have an effect on vaccine uptake.
He told the Andrew Marr Show: "One of the things I'm concerned about in the United States is that part of the anti-science translates maybe into anti-vaccine.
"It would really be a shame if we have a safe and effective vaccine, but a substantial proportion of the people do not want to take the vaccine because they don't trust authority.
"That would really be unfortunate if that's the case."
The Tánaiste @LeoVaradkar says Gov is "increasingly optimistic" that it will be possible to start vaccinating those most at risk "in the first half if not first quarter of next year" @thisweekrte #COVID19— Mary Regan (@MaryERegan) October 25, 2020
Dr Fauci said a potential coronavirus vaccine, alongside public health measures, could help move the US towards "some form of normality" next year.
He said: "I think when we get a vaccine, and we start getting people vaccinated ... over a period of several months into 2021, we will begin to approach some form of normality, depending upon how many people, what proportion of the people, take the vaccine.
"That should be combined with some degree of public health measures.
"I don't think that a vaccine alone right off will get us back to normality.
"What I do foresee is that with a successful vaccine, and the continuation of some form of public health measures, as we go and progress through the months of 2021, getting towards the third and fourth quarter, we will see a considerable approach towards some form of normality."