US government scientist Anthony Fauci said Thursday the coronavirus vaccine "cavalry" is coming but urged the public not to let down its guard, and called for a stronger WHO to ward off future pandemics.

Fauci, a world-leading expert on infectious diseases who has been at serious odds with President Donald Trump over Covid-19, said that after Pfizer's vaccine, another is "literally on the threshold of being announced". 

Moderna, which is co-developing its vaccine candidate with the US National Institutes of Health, says it is close to reaching a threshold in trials to apply for an emergency use authorisation from US regulators.

"The cavalry is coming, but don't put your weapons down," Fauci said by video-link to the Chatham House international affairs think-tank in London.

"Help is on the way, but it isn't here yet," he said, urging the public to respect public health measures such as wearing masks and washing hands.

Fauci added that the coming vaccines should be made available to poorer countries as well: "You should not live or die depending upon where you happen to have been born."

While Trump has sought to sideline Fauci in public, the scientist said he anticipated continuing his four decades of government work, as Democrat Joe Biden prepares to take office in January.

As director of the National Institute of Allergy and Infectious Diseases, Fauci said his role in public health was "getting the message across based on science". 

"I've done that for decades and I think I'm going to be continuing to do that," he said, although for now he is barred from speaking directly to Biden's team as the Trump administration holds out on recognising this month's election result.

Biden has signalled that his administration will reverse Trump's decision to withdraw the United States from the UN's World Health Organization.

The current pandemic was a sharp reminder of the need for global coordination, Fauci said, urging leaders to "make sure the international health structures, the WHO, really get strengthened". 

"It is not a perfect organisation. It has faults that have been pointed out by others. But the world does need a global health organisation."

Fauci also dwelt on the anti-science agitation that has erupted under Trump and other populist leaders, which has seen him personally targeted with death threats.

Last week, former Trump adviser Steve Bannon faced a clampdown from social media companies after he called for the beheading of federal officials including Fauci.

The political and personalised backlash against science was "unprecedented", Fauci said, but he stressed his "fundamental confidence that the better angels will prevail".