As residents return to work after the holiday weekend, concerns turn to a possible second peak in coronavirus cases.

MORE COVERAGE: LIVE BLOG - Coronavirus updates from the tri-state area and beyond 

Memorial Day was the first big test for Americans in the pandemic as states loosened restrictions for the holiday weekend. While some heeded warnings, it seems others did not social distance or wear masks.

Some crowds celebrated closely at beaches in Alabama. In Missouri, a packed pool party has officials urging self-quarantine for 14 days. In Illinois, hundreds defied stay at home orders at a biker bar.

Johns Hopkins University shows the U.S. inching closer to 100,000 coronavirus deaths.

Dr. Mike Ryan, of the World Health Organization, says there may be a second peak within this first wave of cases globally.

"It gives me pause to think about what we may see in the next week or two when people get back to their communities from vacationing down in the Gulf Coast," Dr. Ryan says. "We still have a little bit of ways to go to beat this virus. We're just not there yet, and we have to get people to understand that, or we're going to have a second wave that's going to cost us much more than it already has."

He warns a second peak or wave could come during the normal flu season.

"The disease can jump up any time. We cannot make assumptions that just because the disease is on the way down now it's going to keep going down," Dr. Ryan says.

Another company has also announced a potential coronavirus vaccine is entering human trials. It's the 10th company worldwide to do so.