Dr Anthony Fauci, the United States' top infectious diseases official, said the government wouldn't make any future COVID-19 vaccine obligatory for the general public.
However, local jurisdictions could make it mandatory for some groups, such as children, Dr Fauci said.
"You don't want to mandate and try and force anyone to take a vaccine. We've never done that," said Dr Fauci during a video talk organised by George Washington University.
"You can mandate for certain groups of people like health workers, but for the general population you can't" he added, citing the example of the National Institutes of Health, where health workers can't treat patients without a flu shot.
Prime Minister Scott Morrison hours earlier announced that coronavirus vaccines, once approved, would be "as mandatory as you can possibly make it".
"There are always exemptions for any vaccine on medical grounds but that should be the only basis," he said on Wednesday.
"I mean we’re talking about a pandemic that has destroyed you know, the global economy and taken the lives of hundreds of thousands all around the world and over 450 Australians here."
But the United States' decentralised system of government, and anti-vaccine sentiments that have been building for decades, had in any case made a program of mandatory immunisation unlikely.
"It would be unenforceable and not appropriate," said Dr Fauci.
This would not prevent states from making a vaccine mandatory for children to attend school, as is already the case for certain diseases such as measles, though some are exempt for medical or religious reasons.
The administration of President Donald Trump has pre-ordered hundreds of millions of vaccine doses from six companies, and these will be distributed for free.
People in Australia must stay at least 1.5 metres away from others. Check your state’s restrictions on gathering limits.
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