Revelers in Missouri celebrate Memorial Day weekend at the Lake of the Ozarks on Saturday. (Twitter/Lawler50/Reuters)

The World Health Organization on Tuesday warned nations against scaling back coronavirus restrictions too quickly, saying a premature push to return to normalcy could fuel a rapid acceleration of new cases.

“We cannot make assumptions that just because the disease is on the way down now that it’s going to keep going down,” Mike Ryan, head of the WHO’s health emergencies program, told reporters during a briefing.

He cautioned that countries could face another peak of coronavirus cases even ahead of a presumed second wave of infections months from now. “The disease can jump up at any time,” he added.

That warning echoed concerns from public health experts and others who have urged local, state and federal leaders to move slowly as they seek to reopen their economies. But the pleas for caution collided with a shifting reality as spring gives way to summer.

In the United States, where the death toll on Tuesday approached 100,000, crowds flooded newly reopened beaches and other public spaces over Memorial Day weekend, even as the virus claimed more than 2,000 additional lives.

At a club in Houston, dozens splashed around the pool and sipped on drinks on the patio. In North Carolina, thousands packed the stands shoulder to shoulder at a speedway on opening night, where face masks were the exception. In Daytona Beach, Fla., even after an event called “Orlando Invades Daytona” was canceled, hundreds of people still danced in the street and on top of cars near the boardwalk.

In Missouri, vacationers flocked to the Lake of the Ozarks, packing pools and outdoor bars as they flouted social distancing guidelines. In Maryland, visitors crammed the boardwalk at Ocean City.

The piecemeal return to normal life in the United States, where the number of confirmed coronavirus infections at nearly 1.7 million is more than quadruple any other nation, has unfolded unevenly and uneasily as different states have taken different approaches.

The raucous events across the country over the holiday weekend led some local officials to express alarm that the consequences could be dire if such behavior continues unchecked.

Houston Mayor Sylvester Turner (D), for instance, pledged to crack down on businesses failing to enforce capacity restrictions. Turner chastised clubgoers whose behavior risks exposing residents “who chose to do the right thing” by staying home.

After images of the partyers at the Lake of the Ozarks went viral over the weekend — including a pool party at Backwater Jack’s that featured the theme “Zero Ducks Given” — St. Louis County officials issued a travel advisory, calling the scenes an “international example of bad judgment.”

The weekend’s crowded festivities in parts of the country could foreshadow the challenges local officials increasingly will face this summer as governors gradually roll back restrictions and residents flood reopened businesses.

In areas that no longer have enforceable executive orders, authorities insist there is little they can do to require people to practice social distancing. The mayor of Osage Beach, Mo., where the packed pool parties took place, said he views it as essential that his town’s tourist-dependent businesses reopen, while police there said they couldn’t enforce restrictions.

“It kind of ties our hands when they’re just guidelines and not mandates,” Chris Twitchel, captain of operations for the Camden County Sheriff’s Office, told The Washington Post over the weekend.

Elsewhere, even where executive orders remain in place, some local authorities have refused to enforce them, and many residents have chosen to ignore them. Such was the case in Alamance County, N.C., where an estimated 4,000 spectators filled the stands for races at Ace Speedway.

Although Democratic Gov. Roy Cooper’s second phase of reopening North Carolina limits outdoor gatherings to 25 people, Alamance County Sheriff Terry Johnson said he would refuse to “enforce an unconstitutional law,” allowing the speedway’s opening night to proceed on Saturday, WTVD reported.

“We’re tired of being stuck in the house,” one spectator told a local news station. “I’m not afraid of this virus one bit.”

Several factors have contributed to ongoing clashes about how long restrictions should remain in place. Some parts of the country continue to wrestle with crippling caseloads, while others have barely been affected by the pandemic. Whether to wear masks has become a political issue, rather than merely a public health one. And on top of those divides, much about the virus remains uncertain and mysterious.

“What’s crazy is, we’re three months in, and we’re still not able to calibrate our risk management. It’s a mess,” said University of Virginia professor Brian Nosek, who runs the Center for Open Science, which advocates for transparency in research.

Basic questions about the virus that causes the disease covid-19 remain unanswered: How deadly is it? Exactly how contagious? Are there different strains with different clinical outcomes? Why does the virus create a devastating disease in some people while leaving others with few, if any, symptoms?

With stay-at-home orders around the country expiring and businesses reopening, all the scientific data is being scrutinized anew. But the numbers are often ambiguous, with large margins of error. Because this is still an early phase of the pandemic, scientific findings have to be couched in tentative, provisional, sometimes squishy language festooned with caveats and admitted limitations.

Despite that lingering uncertainty and the crowds in recent days that have caused angst among public health experts, there were flickers of economic optimism as the week began.

Investors celebrated the reopening of the New York Stock Exchange by sending stocks soaring Tuesday. The Dow Jones industrial average closed the day up 530 points, about 2.2 percent, while the Standard & Poor’s 500 index jumped 36 points, about 1.2 percent, to finish at 2,991.77.

“With more signs of the worst of the virus being behind us, investors are beginning to focus on more countries reopening and the lifting of travel bans around the world,” Torsten Slok, chief economist for Deutsche Bank Securities, said in an email. “More signs of reopening and more signs of travel bans being lifted creates more clarity for markets.”

In Europe, officials faced mounting calls to reopen European Union borders ahead of the summer holiday season, although key details remain unresolved.

Spanish officials will no longer require international tourists to self-quarantine starting July 1, following similar decisions by Italy and Greece.

British Prime Minister Boris Johnson announced that as of June 1, outdoor markets will open, along with car showrooms. Beginning June 15, department stores and independent shops will be allowed to reopen as long as measures to protect staff and customers are in place.

Meanwhile, Palestinian officials on Tuesday reopened one of Christianity’s most sacred sites, Bethlehem’s Church of the Nativity, after it closed to visitors almost two months ago.

More than 1.5 million people passed through U.S. airports during Memorial Day weekend, marking a significant increase from recent weeks — even though that figure represents less than 13 percent of air travel during the same period last year. The number of travelers Thursday and Monday also topped 300,000. It has been more than two months since federal workers last screened that volume of travelers.

New-home sales also grew unexpectedly in April, eking out a slight gain instead of the steep decline that had been forecast, the U.S. Commerce Department reported Tuesday.

Sales of new single-family homes rose 0.6 percent in April — a modest but significant advance given the nearly 40 million job losses the nation has endured since the coronavirus took hold about 10 weeks ago.

Between tweets on Tuesday railing against mail-in ballots and raising a conspiracy theory involving MSNBC host Joe Scarborough, President Trump took time to laud his administration’s response to the pandemic and trumpet the stock market gains.

“States should open up ASAP,” he tweeted. “The Transition to Greatness has started, ahead of schedule.”

In reality, it’s a transition that remains fragile.

“We now see a trend in an uptick in hospitalizations. It’s a small uptick, but it is an uptick, and it is unmistakable, and it is probably a result of reopening,” former Food and Drug Administration commissioner Scott Gottlieb said Tuesday morning in an interview on CNBC. “We expected cases to go up and hospitalizations to bump up when we reopened.”

States such as Florida, Georgia and Virginia have registered increased hospitalization numbers, he said. “We’re going to have to watch it,” Gottlieb said. “The hope is that there’s a seasonal effect. … That seasonal effect will hopefully offset the increased social interaction, which is going to cause cases to go up.”

Social interaction should be approached with some level of caution, he added, referring to the crowds of people celebrating in the Ozarks over the weekend.

“I’m concerned that there are people who think that this is the all-clear,” he said. “I think what we really need to be doing is defining a new normal. We’re going to need to live differently until we get to a vaccine.”

Derek Hawkins, Siobhán O’Grady, Jennifer Hassan, Lateshia Beachum, Joel Achenbach, Taylor Telford, Thomas Heath and Marissa Iati contributed to this report.

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