The United States recorded its 10 millionth case of the coronavirus Monday, according to a count by Johns Hopkins University, the same day that Pfizer and BioNTech announced their vaccine showed 90 percent effectiveness.
Shortly before 1400 GMT Monday the tracker by the Baltimore-based university showed 10,018,278 cases recorded in the United States since the pandemic began, and 237,742 deaths.
Both are the highest tolls in absolute terms in the world.
The United States is now well into its third and by far biggest surge of its epidemic.
But interim results provided by Pfizer and BioNTech offered hope for a game changer in the trajectory of the pandemic.
According to preliminary findings, protection in patients was achieved seven days after the second of two vaccine doses, and 28 days after the first.
Outgoing President Donald Trump, heavily criticised for his handling of the health crisis, described the breakthrough as "great news," while top US pandemic expert Anthony Fauci said the result was "extraordinary."
President-elect Joe Biden meanwhile urged Americans Monday to wear masks to prevent the spread of coronavirus and not consider it a "political statement," as he named the scientists who will lead his administration's response to the pandemic.
"We're still facing a very dark winter," Biden said. "The bottom line: I will spare no effort to turn this pandemic around once we're sworn in on January 20."
Biden applauded the vaccine announcement, but insisted a mask still remains "the most potent weapon" against the virus, and that "today's news does not change that urgent reality."