The news comes amid growing concerns that Europe’s coronavirus cases could spike as returning holidaymakers make their way back home and children returning to school.
In the United States, Dr. Anthony Fauci said that the government would not make it mandatory for the general public to have any future Covid-19 vaccine.
But he said local jurisdictions could make it mandatory for some groups, like children.
"You don't want to mandate and try and force anyone to take a vaccine. We've never done that," said Fauci, a member of the White House coronavirus task force, during a video talk Wednesday organised by George Washington University.
His comments came hours after Australia's Prime Minister Scott Morrison announced that coronavirus vaccines, once approved, would be mandatory for everyone in his country barring medical exemptions.
An ethics and safety debate is raging around the world as the race to develop a vaccine gathers pace.