https://www.bbc.com/news/world-us-canada-53899908
Speaking to Reuters news agency, Dr Anthony Fauci also said doing so could hurt the development of other vaccines.
US President Donald Trump is reportedly considering plans to put out a vaccine before it has been fully tested.
Such a move could boost his chances of re-election in November's presidential election.
Democrats accuse the US president of being prepared to endanger American lives for political gain.
On Saturday, President Trump tweeted that the US Food and Drug Administration (FDA) "is making it very difficult for drug companies to get people in order to test the vaccines".