Studies Show N.Y. Outbreak Originated in Europe

The C.D.C. outlined how essential employees can go back to work even if they have been exposed. Scientists warn that the virus might not fade in warm weather.

This briefing has ended. Follow our live national updates and global coverage on the coronavirus epidemic.

Coronavirus in New York came mainly from Europe, studies show.

New research indicates that the coronavirus began to circulate in the New York area by mid-February, weeks before the first confirmed case, and that it was brought to the region mainly by travelers from Europe, not Asia.

“The majority is clearly European,” said Harm van Bakel, a geneticist at Icahn School of Medicine at Mount Sinai, who co-wrote a study awaiting peer review.

A separate team at N.Y.U. Grossman School of Medicine came to strikingly similar conclusions, despite studying a different group of cases. Both teams analyzed genomes from coronaviruses taken from New Yorkers starting in mid-March.

The research revealed a previously hidden spread of the virus that might have been detected if aggressive testing programs had been put in place. On Jan. 31, President Trump barred foreign nationals from entering the country if they had been in China — the site of the virus’s first known outbreak — during the previous two weeks.

Viruses invade a cell and take over its molecular machinery, causing it to make new viruses. An international guild of viral historians ferrets out the history of outbreaks by poring over clues embedded in the genetic material of viruses taken from thousands of patients.

In January, a team of Chinese and Australian researchers published the first genome of the new virus. Since then, researchers around the world have sequenced over 3,000 more. Some are genetically identical to each other, while others carry distinctive mutations.

C.D.C. issues new back-to-work guidelines for essential workers.

The Centers for Disease Control and Prevention published new guidelines on Wednesday detailing how essential employees can go back to work even if they have been exposed to people infected by the coronavirus, provided they do not feel sick and follow certain precautions.

Those employees can return if they take their temperature before heading to their workplaces, wear a face mask at all times and practice social distancing while on the job, Dr. Robert Redfield, the C.D.C. director, said at the White House briefing. They should not share headsets or other objects that touch their faces, and they should not congregate in break rooms or crowded areas, he said.

Dr. Redfield said that employers should send workers home immediately if they developed any symptoms. He also said they should increase air exchange in their buildings and clean common surfaces more often. The goal, he said, was to “get these workers back into the critical work force so that we don’t have worker shortages.”

The new guidance appears to blend earlier advice. Last week, the C.D.C. recommended that even healthy Americans wear masks in public after data showed as many as 25 percent of people infected with the virus were asymptomatic, at the urging of the White House, businesses, workers and others to kick-start the idled economy.

Dr. Deborah Birx, the White House’s coronavirus response coordinator, and other government experts suggested at the briefing that the strict measures being taken by Americans to stem the spread of the virus may be leveling new cases in areas like New York, Detroit, Chicago and Boston.

In New York, ‘the bad news is actually terrible.’

New York, the hardest hit state in America, reported its highest number of coronavirus-related deaths in a single day on Wednesday, announcing that another 779 people had died. That brought the virus death toll to 6,268 in New York State, which Gov. Andrew M. Cuomo noted was more than twice as many people as the state had lost in the Sept. 11, 2001, attacks.

“I went through 9/11,” he said at his daily briefing. “I thought in my lifetime I wouldn’t have to see anything like that again — nothing that bad, nothing that tragic.”

The number of hospitalizations had fallen in recent days, he said, suggesting that social distancing measures were working to flatten the steep curve of the virus’s spread, at least for now. The rates depend not only on the number of new arrivals but also on hospital admission standards.

“If we stop what we are doing, you will see that curve change,” Mr. Cuomo warned.

Then he pivoted to a more somber tone. “The bad news isn’t just bad,” he said. “The bad news is actually terrible.”

New Jersey also had a record number of deaths in the past day: Gov. Philip D. Murphy said that 275 people had died there, up from 232 on Tuesday. More people have died in New York and New Jersey — a total of 7,772 — than in the rest of the United States combined.

Mr. Cuomo said that the staggering death toll could continue to rise even as hospitalization rates were falling, because it reflected people who had been on ventilators for long periods of time.

He expressed reluctance to offer a timeline on when social gatherings could begin again, when he was asked about New York’s theater industry, which will remain shuttered until at least June. “I wouldn’t use what Broadway thinks as a barometer of anything,” he said.

New York State now has more confirmed cases than any single country in the world outside of the United States.

The virus might not fade in warm weather, scientists warn.

The homebound and virus-wary across the Northern Hemisphere, be it Mr. Trump or cooped-up schoolchildren, have clung to the possibility that the pandemic will fade in hot weather, as some viral diseases do.

But the National Academy of Sciences, in a public report sent to the White House, has said, in effect: Don’t get your hopes up. After reviewing a variety of research reports, the panel concluded that the number of studies, of varying quality of evidence, simply do not offer a clear forecast of what will happen to the spread of the coronavirus in the summer.

The report cited a small number of well-controlled laboratory studies that show that high temperature and humidity can diminish the ability of the virus to survive in the environment. But the report noted the studies had limitations that made them less than conclusive.

“Given that countries currently in ‘summer’ climates, such as Australia and Iran, are experiencing rapid virus spread, a decrease in cases with increases in humidity and temperature elsewhere should not be assumed,” the report stated.

Homes for disabled people experience a surge in coronavirus cases.

As the coronavirus preys on the most vulnerable, it is taking root in New York’s sprawling network of group homes for people with special needs. As of Monday, 1,100 developmentally disabled residents in New York State have tested positive for coronavirus, and 105 have died, state officials said, a death rate far higher than in the general population.

Separately, a study by a large consortium of private service providers found that residents of group homes and similar facilities in New York City and surrounding areas were 5.34 times more likely than the general population to develop Covid-19 and 4.86 times more likely to die from it.

A caregiver on Staten Island, who said about 50 colleagues had tested positive, described the challenges faced by those remaining on the job.

“One of the individuals here is positive, and his behavior is to get up, to pace, and he wants to give me a hug, shake my hand,” the caregiver said, asking that his name not be used because he was not authorized to speak.

“They have a hard time realizing that they need to be isolated, and the psychologists aren’t coming out and talking to him,” he added. “We don’t have training for this. We’re just learning on the fly.”