That had been previously demonstrated in laboratory experiments, but now Chinese scientists studying real-world conditions report that they captured tiny droplets containing the genetic markers of the virus from the air in two hospitals in Wuhan, China, where the outbreak started.
Their findings were published Monday in the journal Nature.
It remains unknown if the virus in the samples they collected was infectious, but droplets that small, which are expelled by breathing and talking, can remain aloft and be inhaled by others.
“Those are going to stay in the air floating around for at least two hours,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who was not involved with the Nature paper. “It strongly suggests that there is potential for airborne transmission.”
Dr. Marr and many other scientists say evidence is mounting that the coronavirus is being spread by tiny droplets known as aerosols. The World Health Organization has so far downplayed the possibility, saying that the disease is mostly transmitted through larger droplets that do not remain airborne for long, or through the touching of contaminated surfaces.
Even with the new findings, the issue is not settled. Although the coronavirus RNA — the genetic blueprint of the virus — was present in the aerosols, scientists do not know yet whether the viruses remain infectious or whether the tests just detected harmless virus fragments.
When the government of Sweden defied conventional wisdom and refused to order a wholesale lockdown to “flatten the curve” of the coronavirus epidemic, public health officials pointed to trust as a central justification.
And, to a large extent, Sweden seems to have been as successful in controlling the virus as most other nations. The country’s death rate of 22 per 100,000 people is the same as that of Ireland, which has earned accolades for its handling of the pandemic.
But on one warm spring day in Stockholm last week, there was little evidence that people were observing the protocols. Young Swedes thronged bars, restaurants and parks, drinking in the sun.
While other countries were slamming on the brakes, Sweden kept its borders open, left schools in session and placed no limits on public transport. Hairdressers, gyms and some cinemas have remained open.
Gatherings of more than 50 people were prohibited, and at the end of March, the authorities banned visits to nursing homes. But there are almost no fines, and pedestrians wearing masks are generally stared at as if they have just landed from Mars.
Throughout the crisis, Sweden has had enough intensive care units to deal with Covid-19 patients, said the minister of health and social affairs, Lena Hallengren. “We have 250 empty beds right now.”
This is not to say that Sweden has escaped Covid-19’s deadly consequences. The Swedish Public Health Authority has admitted that the country’s seniors have been hit hard, with the virus spreading through 75 percent of the 101 care homes in Stockholm.
The country’s state epidemiologist, Anders Tegnell, acknowledged that Sweden will have to face its broad failing with people over the age of 70, who have accounted for a staggering 86 percent of the country’s 2,194 fatalities to date.
The public health authority also announced last week that more than 26 percent of the 2 million inhabitants of Stockholm will have been infected by May 1.
But even that figure was presented as something of a win: a number of infections that might limit future outbreaks, reached without suffering an inordinate number of deaths.
Even as the number of coronavirus cases in the United States passed one million on Tuesday, President Trump and some businesses sought to ease restrictions that have severely limited daily life.
Mr. Trump on Tuesday night signed an executive order declaring meat processing plants “critical infrastructure,” and earlier this week told governors to consider reopening school districts before the end of the academic year. In the private sector, the country’s largest operator of malls has developed a plan to reopen 49 shopping centers across 10 states starting on Friday.
Still, many facets of American life seem sure to be crippled into the summer. Even with Mr. Trump’s encouragement, and weeks of growing impatience from parents and children, few governors are considering the idea of reopening schools before summer. Gov. Gavin Newsom of California did raise the idea that the next academic year might begin in July to make up for lost time.
Early returns to pre-pandemic routines will surely be substantially altered, as health officials have stressed that returning too quickly to cluttered school hallways or grocery store aisles could set off another round of rapid infections.
Simon Property Group, the mall operator, hopes its game plan for reopening shopping centers will be enough to fend off that possibility. When the centers begin reopening on Friday, security officers and employees will “actively remind and encourage shoppers” to stay away from others and to refrain from shopping in groups. Food court seating will be spaced apart, and reusable trays will be banished. In restrooms, every other sink and urinal will be taped off.
A memo from the company provides a glimpse of how the broader American shopping experience is likely to look as the country begins to slowly reopen. But the success of such an approach depends largely on whether retailers will also decide to reopen stores and whether the public will feel comfortable going to malls when tests for the virus remain difficult to get.
Singapore has seen a surge of coronavirus cases among migrant workers, after months of successfully controlling the outbreak. As of Tuesday, coronavirus cases linked to migrant worker dormitories accounted for 88 percent of Singapore’s 14,446 cases, including more than 1,400 new cases in a single day.
Many migrant workers live on the outskirts of the city in dormitories that can house up to 20 people per room, making it almost impossible to follow social distancing guidelines.
Singapore has traced the contacts of people infected with the coronavirus and released detailed information about clusters of cases. An analysis of the data shows how the virus has spread rapidly among migrant worker dormitories.
The government has directed all laborers living in dormitories to stop working until May 4, imposing a stay-at-home order for 180,000 foreign workers in the construction sector. The government has also declared 25 dormitories as isolation areas, where workers are confined to their rooms.
Transient Workers Count Too, an advocacy group for migrant workers in Singapore, criticized the plan to quarantine such a large population together, comparing the lockdown to situations on cruise ships in which cases multiplied uncontrollably even when passengers were kept to their rooms.
More than 20 percent of Singapore’s population of 5.7 million are foreign workers. Many come from Bangladesh and India, and they work in construction, shipping, manufacturing and domestic service sectors.
The breakdown of nationalities among the confirmed cases shows that workers from these countries have been disproportionately affected.
Warning that Russia has now entered the hardest stage of the coronavirus pandemic, President Vladimir V. Putin on Tuesday extended until May 11 a nationwide lockdown that he first ordered in March.
Mr. Putin told a teleconference of regional governors that Russia had put a brake on the coronavirus outbreak but not yet reached the peak of infection, which means it needs to prolong restrictions on movement across much of the country.
“We must be very disciplined to stop the wave,” he said, assuring Russians that “the more rigidly the rules are observed the faster quarantine can be relaxed.”
The president told regional leaders to decide for themselves what specific lockdown restrictions were needed, while stressing that after May 11 controls could begin to be eased but only slowly.
He warned of a “long and difficult path ahead,” and said the country “cannot afford to lose all we have achieved in recent weeks.”
Russia, which was hit by the virus later than most countries despite its long border with China, still has relatively few cases. It has reported a total of 93,558 confirmed coronavirus infections on Tuesday and just 867 deaths, compared with a death toll of more than 50,000 in the United States.
Reporting was contributed by Kenneth Chang, Thomas Erdbrink, Christina Anderson, Iliana Magra, Nicholas Bogel-Burroughs, Andrew Higgins, Weiyi Cai and K.K. Rebecca Lai.