Skeptics of the notion that the coronavirus spreads through the air — including many expert advisers to the World Health Organization — have held out for one missing piece of evidence: proof that floating respiratory droplets called aerosols contain live virus, and not just fragments of genetic material.
Now a team of virologists and aerosol scientists has produced exactly that: confirmation of infectious virus in the air.
“This is what people have been clamoring for,” said Linsey Marr, an expert in airborne spread of viruses who was not involved in the work. “It’s unambiguous evidence that there is infectious virus in aerosols.”
A research team at the University of Florida succeeded in isolating live virus from aerosols collected at a distance of seven to 16 feet from patients hospitalized with Covid-19 — farther than the six feet recommended in social distancing guidelines.
The findings, posted online last week, have not yet been vetted by peer review, but have already caused something of a stir among scientists. “If this isn’t a smoking gun, then I don’t know what is,” Dr. Marr tweeted last week.
But some experts said it still was not clear that the amount of virus recovered was sufficient to cause infection.
The research was exacting. Aerosols are minute by definition, measuring only up to five micrometers across; evaporation can make them even smaller. Attempts to capture these delicate droplets usually damage the virus they contain.
“It’s very hard to sample biological material from the air and have it be viable,” said Shelly Miller, an environmental engineer at the University of Colorado Boulder who studies air quality and airborne diseases.
“We have to be clever about sampling biological material so that it is more similar to how you might inhale it.”
Previous attempts were stymied at one step or another in the process. For example, one team tried using a rotating drum to suspend aerosols, and showed that the virus remained infectious for up to three hours. But critics argued that those conditions were experimental and unrealistic.
Other scientists used gelatin filters or plastic or glass tubes to collect aerosols over time. But the force of the air shrank the aerosols and sheared the virus. Another group succeeded in isolating live virus, but did not show that the isolated virus could infect cells.
In the new study, researchers devised a sampler that uses pure water vapor to enlarge the aerosols enough that they can be collected easily from the air. Rather than leave these aerosols sitting, the equipment immediately transfers them into a liquid rich with salts, sugar and protein, which preserves the pathogen.
“I’m impressed,” said Robyn Schofield, an atmospheric chemist at Melbourne University in Australia, who measures aerosols over the ocean. “It’s a very clever measurement technique.”
As editor of the journal Atmospheric Measurement Techniques, Dr. Schofield is familiar with the options available, but said she had not seen any that could match the new one.
The researchers had previously used this method to sample air from hospital rooms. But in those attempts, other floating respiratory viruses grew faster, making it difficult to isolate the coronavirus.
This time, the team collected air samples from a room in a ward dedicated to Covid-19 patients at the University of Florida Health Shands Hospital. Neither patient in the room was subject to medical procedures known to generate aerosols, which the W.H.O. and others have contended are the primary source of airborne virus in a hospital setting.
The team used two samplers, one about seven feet from the patients and the other about 16 feet from them. The scientists were able to collect virus at both distances and then to show that the virus they had plucked from the air could infect cells in a lab dish.
The genome sequence of the isolated virus was identical to that from a swab of a newly admitted symptomatic patient in the room.
The room had six air changes per hour and was fitted with efficient filters, ultraviolet irradiation and other safety measures to inactivate the virus before the air was reintroduced into the room.
That may explain why the researchers found only 74 virus particles per liter of air, said John Lednicky, the team’s lead virologist at the University of Florida. Indoor spaces without good ventilation — such as schools — might accumulate much more airborne virus, he said.
But other experts said it was difficult to extrapolate from the findings to estimate an individual’s infection risk.
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Frequently Asked Questions
Updated August 12, 2020
Can I travel within the United States?
- Many states have travel restrictions, and lots of them are taking active measures to enforce those restrictions, like issuing fines or asking visitors to quarantine for 14 days. Here’s an ever-updating list of statewide restrictions. In general, travel does increase your chance of getting and spreading the virus, as you are bound to encounter more people than if you remained at your house in your own “pod.” “Staying home is the best way to protect yourself and others from Covid-19,” the C.D.C. says. If you do travel, though, take precautions. If you can, drive. If you have to fly, be careful about picking your airline. But know that airlines are taking real steps to keep planes clean and limit your risk.
I have antibodies. Am I now immune?
- As of right now, that seems likely, for at least several months. There have been frightening accounts of people suffering what seems to be a second bout of Covid-19. But experts say these patients may have a drawn-out course of infection, with the virus taking a slow toll weeks to months after initial exposure. People infected with the coronavirus typically produce immune molecules called antibodies, which are protective proteins made in response to an infection. These antibodies may last in the body only two to three months, which may seem worrisome, but that’s perfectly normal after an acute infection subsides, said Dr. Michael Mina, an immunologist at Harvard University. It may be possible to get the coronavirus again, but it’s highly unlikely that it would be possible in a short window of time from initial infection or make people sicker the second time.
I’m a small-business owner. Can I get relief?
- The stimulus bills enacted in March offer help for the millions of American small businesses. Those eligible for aid are businesses and nonprofit organizations with fewer than 500 workers, including sole proprietorships, independent contractors and freelancers. Some larger companies in some industries are also eligible. The help being offered, which is being managed by the Small Business Administration, includes the Paycheck Protection Program and the Economic Injury Disaster Loan program. But lots of folks have not yet seen payouts. Even those who have received help are confused: The rules are draconian, and some are stuck sitting on money they don’t know how to use. Many small-business owners are getting less than they expected or not hearing anything at all.
What are my rights if I am worried about going back to work?
What is school going to look like in September?
- It is unlikely that many schools will return to a normal schedule this fall, requiring the grind of online learning, makeshift child care and stunted workdays to continue. California’s two largest public school districts — Los Angeles and San Diego — said on July 13, that instruction will be remote-only in the fall, citing concerns that surging coronavirus infections in their areas pose too dire a risk for students and teachers. Together, the two districts enroll some 825,000 students. They are the largest in the country so far to abandon plans for even a partial physical return to classrooms when they reopen in August. For other districts, the solution won’t be an all-or-nothing approach. Many systems, including the nation’s largest, New York City, are devising hybrid plans that involve spending some days in classrooms and other days online. There’s no national policy on this yet, so check with your municipal school system regularly to see what is happening in your community.
“I’m just not sure that these numbers are high enough to cause an infection in somebody,” said Angela Rasmussen, a virologist at Columbia University in New York.
“The only conclusion I can take from this paper is you can culture viable virus out of the air,” she said. “But that’s not a small thing.”
Several experts noted that the distance at which the team found virus is much farther than the six feet recommended for physical distancing.
“We know that indoors, those distance rules don’t matter anymore,” Dr. Schofield said. It takes about five minutes for small aerosols to traverse the room even in still air, she added.
The six-foot minimum is “misleading, because people think they are protected indoors and they’re really not,” she said.
That recommendation was based on the notion that “large ballistic cannonball-type droplets” were the only vehicles for the virus, Dr. Marr said. The more distance people can maintain, the better, she added.
The findings should also push people to heed precautions for airborne transmission like improved ventilation, said Seema Lakdawala, a respiratory virus expert at the University of Pittsburgh.
“We all know that this virus can transmit by all these modes, but we’re only focusing on a small subset,” Dr. Lakdawala said.
She and other experts noted one strange aspect of the new study. The team reported finding just as much viral RNA as they did infectious virus, but other methods generally found about 100-fold more genetic matter.
“When you do nasal swabs or clinical samples, there is a lot more RNA than infectious virus,” Dr. Lakdawala said.
Dr. Lednicky has received emails and phone calls from researchers worldwide asking about that finding. He said he would check his numbers again to be sure.
But ultimately, he added, the exact figures may not matter. “We can grow the virus from air — I think that should be the important take-home lesson,” he said.