Early this year, as the novel coronavirus began to spread over nearly the entire planet, scientists and public health experts labored under the consensus that the pathogen spread only or at least primarily through close personal contact.
That consensus was likely mistaken, scientists have increasingly concluded in recent months. SARS-CoV-2, the virus that causes COVID-19, appears to be airborne—or at least prone to spreading through “aerosols,” or tiny particles even smaller than the respiratory droplets we were all so spooked about this winter. Now, researchers are hustling to figure out what safety measures ought to look like when the coronavirus could be lurking in the air.
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In the months after SARS-CoV-2 first appeared in Wuhan, China, researchers came to the conclusion that the virus usually spread by riding along on large droplets that are at least five microns in diameter. The kind you exhale when you breathe, talk, sing, or cough.
But the virus probably didn’t travel on the much finer mists, called “aerosols,” that result when those large droplets thin out in the air—or so the thinking went. The distinction between a virus that’s airborne and one that isn’t is whether it can travel on aerosols while remaining infectious.
In other words, you needed to be within several feet of an infected person in order to catch the virus that causes COVID-19. Farther away, and you’d likely be out of reach of the larger, virus-laden droplets—as those droplets “would fall out of the air fairly quickly,” Doug Reed, a University of Pittsburgh immunologist, told The Daily Beast.
Then, over the summer, a growing body of research bolstered previous advocates of the idea of aerosol spread—and definitively challenged that consensus. “Case studies have played a large role in highlighting the limits of our assumptions and learned knowledge as the pandemic grew,” Herek Clack, a University of Michigan engineer who develops medical protective gear, told The Daily Beast.
Scientists noted that pathogens similar to the novel coronavirus—such as SARS-CoV-1 and MERS—could travel on aerosols. “There is every reason to expect that SARS-CoV-2 behaves similarly, and that transmission via airborne microdroplets is an important pathway,” researchers Lidia Morawska and Donald Milton noted in an early-July paper in the journal Clinical Infectious Diseases.
On July 6, 239 scientists from all over the world signed a letter urging the World Health Organization, which they believed was lagging behind, to make it official. “There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several meters, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission,” the scientists wrote.
WHO saw the light—or at least nodded to it. “The physics of exhaled air and flow physics have generated hypotheses about possible mechanisms of SARS-CoV-2 transmission through aerosols,” the organization announced on July 9.
For many experts on aerosols, it was like a starting pistol went off. Before, they’d largely sat on the sidelines. But that “quiet in the airborne transmission protection arena, at least over the first half of the year, likely was attributable to lack of a consensus position on what role airborne transmission plays,” Clack said.
When the consensus began to shift, these experts in aerosols suddenly found themselves on the front lines of coronavirus science. They’ve got their work cut out for them.
Some chilling findings came to light quickly. A team of scientists at the University of Florida recently collected air samples in the room of two COVID-19 patients and found traces of what they described as infectious virus at a distance of up to 16 feet. “If this isn't a smoking gun, then I don't know what is,” tweeted Linsey Marr, a Virginia Tech expert in the airborne spread of viruses who has had something of a star turn in recent months. That study has not yet been peer-reviewed.
But there’s still a lot more work to do to understand how the novel coronavirus might spread through the air.
The “biggest gap” in our understanding, according to Pierre Mourad, a University of Washington bio-engineer, concerns asymptomatic transmission. “Do people positive for SARS-CoV-2 but without obvious respiratory symptoms—for example, coughing—emit aerosols that contain SARS-CoV-2 capable of infecting others?” Mourad asked. “If so, what factors—biological, medical, physical—govern the type… and viral load of these aerosols?”
“Since a large percentage of people positive for the virus are symptom-free, or weakly symptomatic, this is the No. 1 issue.”
Prior to July, the WHO had claimed that asymptomatic transmission was probably “very rare.” But the organization has been aggressively pressured on that front, as well. “There’s a lot of research that needs to be done to really understand this, and we are open to the fact that there is new research every day,” Maria Van Kerkhove, WHO’s technical lead for coronavirus response, told The New York Times.
There’s another big gap in our understanding of how the novel coronavirus might spread on aerosols, Mourad said—and it matters the most for health-care workers. “For those unavoidably exposed to COVID-19 patients with symptoms, under what circumstances do those patients emit what kind of load of infective SARS-CoV-2?”
Scientists also should study whether so-called “super spreaders”—who infect many others—produce more virus-laden aerosols than “normal” coronavirus carriers do, Mourad said. The typical person infected with SARS-CoV-2 infects a handful of other people, meaning it has a so-called “R-naught value”—or reproduction rate—in the low single digits. Super-spreaders might infect double digits.
Mourad said scientists were moving quickly to investigate these and other questions regarding the novel coronavirus and aerosols. The biggest impediment to quick results, he said, was “getting biologists, medical providers, and aerosol experts together with sufficient resources.”
Clack shared that concern. “Those of us who already were working in this area have been incredibly busy, limited by hours in the day, availability of well-trained assistants, and shutdown-related obstacles to research,” he said.
The new aerosol science, whenever we get more of it, could shape public health measures as the United States and other hard-hit countries move into the fall and winter. Mourad posited a worst-case scenario where researchers conclude that transmission-by-aerosol is widespread and far-reaching.
In that case, it’s not inconceivable that safety measures that go above and beyond already-controversial masks begin to proliferate. As ridiculous as it might sound, Mourad proposed a handheld filtering system combining a mask, a vacuum-cleaner, and air filters.
Still, at least one coronavirus-expert warned against panicking over the prospect of airborne SARS-CoV-2.
Yes, it’s possible the novel coronavirus is airborne, Susan Butler-Wu, a clinical microbiologist at the University of Southern California, told The Daily Beast. No, that might not force us to immediately make major changes to existing public health guidance. “We know for sure it’s not predominantly an airborne-driven phenomenon,” Butler-Wu argued.
If the pathogen were mainly airborne, the number of other people an infected person gives the virus to would be much higher than the currently estimated rate, Butler-Wu said. A highly infectious pathogen such as measles has an R-naught of 15 or so.
Health-care workers are the people to whom the aerosol discussion remains most important, Butler-Wu noted. They work with known carriers of the disease in indoor settings. Some clinical procedures, such as intubation—part of putting someone on a ventilator—have long been known to produce lots of aerosols. Medical personnel already tend to wear a lot of protective gear. If SARS-CoV-2 really is airborne, it only reinforces just how important it is for health-care workers to have the gear they need, from masks to face guards, especially as the fall months augur more infection.
While we await new findings on possible airborne transmission of the novel coronavirus, existing public health advice stands.
As Mourad put it, “Wear a mask when you have the opportunity to interact with others not in your bubble!”
