Pfizer said its experimental Covid-19 vaccine was more than 90% effective as it disclosed the first successful interim data from a large-scale clinical test. That was a major breakthrough in a pandemic that has killed more than 237,000 Americans and contributed to Trump's defeat in last week's election.
The drugmaker operated independently of the White House's "Operation Warp Speed" programme that pumped billions of dollars into six other pharmaceutical companies intended to accelerate development of the vaccine, though Vice President Mike Pence on Twitter incorrectly credited a "public-private partnership" with the development.
Biden, who said his advisers had been told of the breakthrough on Sunday night (local time), warned it will still be months before the vaccine is widely available in the United States.
"As we work toward a safe and effective vaccine, we know the single most effective thing we can do to stop the spread of Covid is wear a mask," Biden, who made his criticism of Trump's handling of the pandemic a centerpiece of his campaign, told reporters in Wilmington, Delaware.
Biden spoke two days after clinching election victory over Trump, who has not acknowledged defeat and is pursuing legal challenges to the results while making unsubstantiated claims of fraud.
Trump on Twitter celebrated the breakthrough and a resultant stock market rally: "STOCK MARKET UP BIG, VACCINE COMING SOON. REPORT 90% EFFECTIVE. SUCH GREAT NEWS!"
The United States has been registering record high infection numbers in recent days. Mask wearing has become a political issue in the United States, with Trump mocking Biden for wearing a mask during the campaign and many conservatives contending masks infringe upon their individual freedom.
Trump's oldest son, Donald Trump Jr., without evidence accused pharmaceutical companies of deliberately holding back clinical trial results until after the November 3 election.
"They all knew it but kept it from the public on purpose," Trump Jr. said on Twitter. "Big Pharma hates Trump for taking on the gouging of Americans with drug pricing while offering the same drugs elsewhere in the world for pennies in the dollar."
Biden during the campaign accused Trump of panicking and surrendering to the pandemic. Trump promoted unproven medicines, assailed public health experts, failed to signal empathy or compassion as the death toll mounted and disregarded advice on mask wearing and social distancing, ending up hospitalized in October receiving treatments for Covid-19.
Housing Secretary Ben Carson on Monday became the latest Trump associate to test positive for the virus, his chief of staff said, just days after White House Chief of Staff Mark Meadows also tested positive.
Pfizer and German partner BioNTech SE are the first drugmakers to release successful data from a large-scale clinical trial of a vaccine for the novel coronavirus.
Biden cleared the threshold of 270 Electoral College votes needed to win the presidency on Saturday after four days of ballot counting. He beat Trump by more than 4.3 million votes nationwide, with Trump becoming the first U.S. president since 1992 to lose a re-election bid.
"We can save tens of thousands of lives if everyone would just wear a mask for the next few months. Not Democratic or Republican lives. American lives," Biden said. "So please, I implore you: Wear a mask. Do it for yourself. Do it for your neighbour. A mask is not a political statement, but it is a good way to start pulling the country together."
Trump, apparently intent on settling scores within his administration, on Monday said in a tweet he had "terminated" Defense Secretary Mark Esper. Trump had split with Esper over a range of issues.
