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And if the country had begun locking down cities and limiting social contact on March 1, two weeks earlier than when most people started staying home, a vast majority of the nation’s deaths — about 83 percent — would have been avoided, the researchers estimated.

The enormous cost of waiting to take action reflects the unforgiving dynamics of the outbreak that swept through American cities in early March. Even small differences in timing would have prevented the worst exponential growth, which by April had subsumed New York City, New Orleans and other major cities, the researchers found.

“It’s a big, big difference,” said Jeffrey Shaman, an epidemiologist at Columbia and the leader of the research team. “That small moment in time, catching it in that growth phase, is incredibly critical in reducing the number of deaths.”

The findings are based on infectious-disease modeling that gauges how reduced contact between people starting in mid-March slowed transmission of the virus.

But in cities like New York, where the virus arrived early and spread quickly, those actions were too late to avoid a calamity. Dr. Shaman’s team modeled what would have happened if those same changes had taken place one or two weeks earlier and estimated the spread of infections and deaths until May 3.

The results show that as states reopen — all 50 states had eased restrictions somewhat as of Wednesday — outbreaks can easily get out of control unless officials closely monitor infections and immediately clamp down on new flare-ups.

And they show that each day that officials waited to impose restrictions in early March came at a great cost.

An additional 2.4 million workers filed first-time claims for unemployment benefits last week, the U.S. government reported Thursday, representing a leveling in the weekly toll that the coronavirus lockdowns have had on the economy.

Still, the number of people losing their jobs remains vast. The latest report, for the period ending May 16, brings the total count of jobless claims over the past nine weeks to more than 38 million.

And the pain is widespread. A recent household survey from the Census Bureau found that nearly half of adults said they or a member of their household had lost employment income since mid-March. A recent Federal Reserve study found that approximately 40 percent of workers in households earning less than $40,000 had lost their jobs.

While all 50 states have begun the process of reopening, most experts believe the path to economic recovery will be long and potentially halting. Economists who once expected a swift recovery now say unemployment is likely to remain elevated for years.

Emergency relief and expanded unemployment benefits have helped tide over households. Roughly three-quarters of people who are eligible for a $1,200 stimulus payment from the federal government have received it, according to the Treasury Department.

Workers who have successfully applied for unemployment benefits are getting an extra $600 a week from the federal government, and most states have begun another program that extends benefits to freelancers, self-employed workers and others who don’t routinely qualify.

But many states are struggling to keep up with the overwhelming demand, drawing desperate complaints from people who have been waiting two months or more to receive their first benefit check. Indiana, Wyoming, Hawaii and Missouri are among those with large backlogs of incompletely processed claims. Another is Kentucky, where nearly one in three workers are unemployed.

The coronavirus pandemic has devastated nursing homes in the United States, contributing to at least 20 percent of the nation’s Covid-19 death toll. The impact has been felt in cities and suburbs, in large facilities and small, in poorly rated homes and in those with stellar marks.

A collaboration by reporters at The New York Times, The Baltimore Sun, KPCC/LAist and The Southern Illinoisan illustrates the toll.

More than 60 percent of nursing homes where at least a quarter of the residents are black or Latino have reported at least one coronavirus case, a New York Times analysis shows. That is double the rate of homes where black and Latino people make up less than 5 percent of the population.

In the suburbs of Baltimore, for example, workers at one nursing home said they were given rain ponchos to protect from infection. Twenty-five employees at the facility, where most residents are African-American, tested positive for the coronavirus.

In East Los Angeles, a staff member at a predominantly Latino nursing home where an outbreak emerged said she was given swimming goggles before professional gear could be obtained. She said she later tested positive for the virus.

“It’s still amazing to me, like, how can that be the case, that there is not a more systematic way to address a central need?” said Fyodor Urnov, the scientist who oversaw the transformation of the Innovative Genomics Institute into a clinical laboratory.

There have been calls for more than a decade to create a national laboratory system that could oversee a testing response in a public health crisis. An effort to create one 10 years ago withered away over time because of a lack of funding.

In recent days, President Trump has delivered a mixed message on testing, saying on May 11 that in ramping up, “we have met the moment and we have prevailed,” while a few days later, he suggested that testing was “overrated” and that the high number of cases in the United States could be traced to more prevalent testing.

The picture for testing is slowly improving. The United States is completing more than 300,000 tests a day, double the amount of a month ago, according to the Covid Tracking Project. But the country still needs to triple its capacity to at least 900,000 tests a day, and as many as 20 million tests must be completed in order to get an accurate view of the outbreak, particularly as states ease virus-related restrictions.

The scientist, Moncef Slaoui, is a venture capitalist and a former longtime executive at GlaxoSmithKline. Most recently, he sat on the board of Moderna, a Cambridge, Mass., biotechnology firm with a $30 billion valuation that is pursuing a coronavirus vaccine. He resigned when President Trump named him to the new post as chief adviser for Operation Warp Speed, the federal drive for coronavirus vaccines and treatments.

Just days into his job, the extent of Dr. Slaoui’s financial interests in drug companies has begun to emerge: The value of his stock holdings in Moderna jumped nearly $2.4 million, to $12.4 million when the company released preliminary, partial data from an early phase of its candidate vaccine trial that helped send the markets soaring on Monday.

Dr. Slaoui sold his shares on Tuesday, and the administration said he would donate the increased value to cancer research.

But the Moderna stock is just one piece of his pharmaceutical portfolio, much of which is not public. And some ethics and financial securities experts have voiced concerns about the arrangement Dr. Slaoui struck with the administration.

In agreeing to accept the position, Dr. Slaoui did not come on board as a government employee. Instead, he is on a contract, receiving $1 for his service. That leaves him exempt from federal disclosure rules that would require him to list his outside positions, stock holdings and other potential conflicts. And the contract position is not subject to the same conflict-of-interest laws and regulations that executive branch employees must follow.

Pelosi and Schumer urge Trump to lower flags to half-staff when coronavirus death toll reaches 100,000.

Calling for “a national expression of grief,” the Democratic leaders of the House and Senate asked President Trump on Thursday to order flags on all government buildings be lowered to half-staff when the country reaches 100,000 coronavirus-related deaths in the coming days.

In a letter to Mr. Trump on Thursday morning, Speaker Nancy Pelosi of California and Senator Chuck Schumer of New York, the minority leader, said doing so would be an appropriate gesture at a time when the United States is preparing to honor those it lost to war on Memorial Day.

“Respectful of them and the loss to our country, we are writing to request that you order flags to be flown at half-staff on all public buildings in our country on the sad day of reckoning when we reach 100,000 deaths,” they wrote. “It would serve as a national expression of grief so needed by everyone in our country.”

More than 93,000 people have now died in connection with the virus, according to a tally by The New York Times, and an average of more than 1,000 deaths a day are still being recorded.

Mr. Trump has not led any observance of national mourning since the coronavirus pandemic began claiming American lives by the thousands, and in his recent public comments, he has steered clear of talking about the deaths, focusing instead on the need to reopen the country — a process he describes as a “transition to greatness” — and defending his own handling of the crisis.

Ms. Pelosi can lower flags over the Capitol on her own authority, but it takes an order from the president to lower flags at the White House and on federal buildings around the country.

States and cities around the country have begun with varying degrees of success to ramp up efforts to put contact tracing in place on a large scale. Last week, Gov. Philip D. Murphy of New Jersey said that his state would hire up to several thousand contact tracers to assist the 800 now working for local and county health departments.

Twenty miles to the west of New York City, Paterson, a poor, largely nonwhite city of about 150,000, has been tracing the virus at a level that could be the envy of larger cities. The team has been able to successfully investigate and trace about 90 percent of the more than 5,900 positive Covid-19 cases in Paterson, said the city’s top health officer, Dr. Paul Persaud.

As of Saturday, 306 Paterson residents have died, giving the city a Covid-19 death rate of 5.1 percent among those who have tested positive for the disease, compared to 7 percent statewide.

Perry N. Halkitis, the dean of the Rutgers School of Public Health, said that it was impossible to know how much contact tracing has helped control the spread of the virus. But contact tracing, he noted, “is one of the few tools that we actually have in the absence of a vaccine.”

Michigan, one of the states hardest hit by the pandemic, was confronted by a new emergency on Wednesday after days of torrential rainfall breached two dams the night before. Thousands of residents were forced to flee their homes, and much of Midland, the home of Dow Chemical and some of its plants, was submerged.

“It’s hard to believe that we’re in the middle of a 100-year crisis, a global pandemic, and we’re also dealing with a flooding event that looks to be the worst in 500 years,” Ms. Whitmer said.

For years, federal regulators had warned that a dam in nearby Edenville Township could rupture and had chided its corporate owner, Boyce Hydro Power, for failing to make required structural changes. On Tuesday night, the dam gave way, sending water gushing into streets and threatening Dow Chemical, the producer of plastics that sits along the Tittabawassee River. Ten miles south of the Edenville dam, water was spilling over a second dam, a structure feared to be on the verge of collapse on Wednesday.

By then, floodwaters had crept high enough that red stop signs were barely peeking out in downtown Midland, a city of 42,000 residents about 130 miles northwest of Detroit.

Officials said there were no known injuries or deaths tied to the floods.

As news of the disaster spread, Mr. Trump threatened on Twitter to withhold federal funds to Michigan if the state proceeded to expand vote-by-mail efforts. The president then followed up by saying that the Federal Emergency Management Agency and the military had been deployed to Michigan to assist with disaster response.

Baltimore’s ‘Squeegee Kids’ stay on the job: ‘I am going to work.’

With a squeegee in his left latex-gloved hand and a plastic spray bottle with a solution of vinegar, water and glass cleaner in his right, a young man stood alone on a Baltimore street corner and waited for the traffic lights to turn red.

As soon as it did, he tried to clean windshields for a small donation.

The young man, Evay, who asked that his last name be withheld, was a food runner and busser at a restaurant on Baltimore’s Harbor East front. But like many dining establishments around the country, it was forced to shutter to slow the spread of the coronavirus.

Now Evay relies on washing car windows to pay for food and rent. Before the virus hit, he could make almost $200 cleaning the windows, but now, on a good day, he makes half that.

While Gov. Larry Hogan of Maryland, a Republican, has started reopening parts of the state, Baltimore’s mayor, Bernard C. “Jack” Young, a Democrat, has said that the city will keep its stay-at-home order in place for now.

Mr. Young also said this week that the city’s summer jobs program for young people would have to be cut in half, and offer many jobs virtually.

Many of Baltimore’s squeegee workers aren’t in high school, so they don’t qualify for the free lunches made available to the community by local school districts.

The Mayor’s Office of Children and Family Success is trying to help by hiring squeegee workers to deliver the free meals. The initiative is part of the “Earn As You Grow” piece of what the office calls the Squeegee Alternative Plan. The pay is $50 a day, or $250 per week.

Washing car windows at intersections is not illegal in Baltimore, but some city officials have tried to ban it. The police aren’t targeting the workers during the lockdown, one officer said, though they may be asked to disperse if they are in large groups.

Evay said an officer asked him to move, telling him he was not supposed to be outside unless he was going to work. “I said, ‘I am going to work.’”

In many states, the beaches are open or opening for Memorial Day weekend. Exciting news, but no activity is going to be risk-free in a pandemic. Here are some things to remember when planning a beach day: know the rules, consider your beach’s geography, stay moving or far away, use your own gear and check out the restroom facilities when you arrive.

Global updates: China imposes a quarantine in its northeast.

The latest outbreak of coronavirus cases in China is concentrated in Jilin, a northeastern province of 27 million people that sits near the borders with Russia and North Korea. Jilin has reported only about 130 cases and two deaths, but experts there have warned of a potential “big explosion.”

Tips for biking as a family.

The humble bicycle is the surprise star of lockdown. With youth sports on hold, car traffic down 75 percent or more throughout the United States (according to the research firm StreetLightData), and cooped-up children doing parkour on the living room furniture, family bike rides have never sounded better. Here are some tips for a safe and successful trip.

Reporting was contributed by Karen Barrow, Eileen Sullivan, Julie Bosman, Katie Thomas, Patricia Cohen, Andrew Das, James Glanz, Matthew Goldstein, Abby Goodnough, Kathleen Gray, Maggie Haberman, Sheila Kaplan, Sharon Otterman, Campbell Robertson, Chaseedaw Giles, Anna Schaverien, Lauren Sloss, Kaly Soto, Chris Stanford and Alexandra Stevenson.