US coronavirus cases crossed the 15 million mark on Tuesday as regulators moved a step closer to approving a Covid-19 vaccine and President-elect Joe Biden laid out his plan to fight the pandemic during his first 100 days in office.

Record cases in at least three states - Arizona, Alabama and Ohio - pushed the cumulative case load to over 15 million, according to a Reuters tally of state and county data. With the virus showing no sign of abating, leading health officials are once again sounding the alarm of further spread when people gather for the year-end holidays.

"We're in for a very challenging period," top infectious disease expert Dr. Anthony Fauci told a virtual summit on Tuesday.

At a briefing in Wilmington, Delaware, Biden said he needed Congress to fully fund delivering vaccines to all corners of the United States. Getting children back to school will be a national priority in the first 100 days, Biden said.

"In 100 days, we can change the course of the disease and change life in America for the better," said Biden, who takes office on Jan. 20. "Whatever your politics or point of view, mask up for 100 days."

US President Donald Trump signed an executive order on Tuesday, which his administration says will ensure that "Americans receive priority access" to the new coronavirus vaccines procured by the US government before they are made available to other nations.

A senior administration official confirmed that the vaccine will be given for free to US citizens.

In Europe, Germany was battling a second virus wave much more deadly than the first yesterday with record fatalities.

German Chancellor Angela Merkel is urging regions to take much more drastic action to curb the spread, after state leaders agreed measures that would see comparatively modest social restrictions eased even further for the Christmas holidays.