Is the coronavirus airborne? Evidence is scant, infectious disease experts say

Social distancing rules limit seating in Singapore in March 2020.
Social distancing rules limit seating in Singapore in March 2020. (Image credit: Shutterstock)

Experts agree that the novel coronavirus behind COVID-19 spreads through direct contact and large droplets that, once sneezed or coughed out, often fall harmlessly to the ground. But a recent letter signed by 239 scientists suggests that the virus may also spread by airborne transmission, lingering in the air for hours within lighter "microdroplets." The letter challenges the most recent guidelines of the World Health Organization (WHO), urging the institution  to recognize airborne transmission as a significant route of COVID-19 infection, and plan accordingly.

"We need to be attentive and mindful of all the important transmission pathways to make progress" with COVID-19, William Nazaroff, contributing author of the letter and professor emeritus of civil and environmental engineering at the University of California Berkeley, told Live Science.

On Tuesday (July 7), WHO said it is reviewing new evidence on whether COVID-19 can spread through airborne transmission, and would provide an update on the topic in the coming days, according to CNBC.

But epidemiologists and infectious disease experts are wary of the limited evidence for airborne transmission, and concerned that recent media reports on this letter will do more harm than good. "It is a shame that they felt the need to publish," Paul Hunter, a professor at the University of East Anglia in the United Kingdom and a member of WHO's infection prevention committee, told Live Science. What's more, even if COVID-19 can spread via the airborne route, it's likely this happens only in limited circumstances and doesn't require additional precautions in most cases, experts said.

"Given the ample evidence that reducing droplet transmission works [to reduce COVID-19 spread], throwing other things into the mix only confuses people and undermines the World Health Organization at a critical time," Hunter said.

The strongest evidence for airborne transmission of the novel coronavirus, SARS-CoV-2, is prior work suggesting that the related virus behind the 2003 SARS epidemic spread through airborne transmission. "There is every reason to expect that SARS-CoV-2 behaves similarly, and that transmission via airborne microdroplets is an important pathway," according to the letter, published Monday in the journal Clinical Infectious Diseases

Nazaroff and colleagues (including Lidia Morawska of Queensland University of Technology in Australia, the lead author of the open letter) also point to a COVID-19 "superspreading" event that occured at a choir in Washington in early March as evidence of airborne transmission. In a separate article currently under review, the authors describe the recent outbreak in which 53 of 61 members attending a weekly choir rehearsal contracted COVID-19, and conclude that airborne microdroplets were the most likely mode of transmission.

"Contortionist thinking is required to explain this event by anything other than inhalation of the shared air," Nazaroff said. "I'm not an epidemiologist. I'll just say that I've not seen persuasive presentations that more than 11 million diagnosed infections can be accounted for by a combination of droplet, close contact transmission, and fomite [surface] transmission."

If Nazaroff is right, then there may be a need for special N95 respirator masks beyond the standard surgical masks used by healthcare providers. Social distancing as it is currently practiced may be insufficient. Businesses may need to revamp their ventilation systems by eliminating air recirculation and supplementing existing ventilation with portable air cleaners. 

But for epidemiologists, the question is not whether airborne transmission is theoretically possible, or even whether it has occurred in isolated cases. The question is whether airborne microdroplets are a significant pathway for infection — significant enough to warrant changes in WHO guidelines and major adjustments to masking and ventilation protocols.   

There, the evidence is less compelling, experts say. 

"When infectious disease physicians think about airborne transmission, we are not talking about an aerobiological experiment. We are looking for the force of transmission that is driving the epidemiology" of an outbreak, Dr. Amesh Adalja, an infectious disease specialist at Johns Hopkins University, told Live Science. 

While Adalja allows that the novel coronavirus may indeed spread through airborne microdroplets, he does not think that this is a significant mode of transmission. "With measles, we know that someone can cough in an elevator and, thirty minutes later, that air is still infectious even with fleeting contact," he said. This is not what experts have seen with COVID-19. "It is mostly droplet transmission. You might be able to demonstrate that some aerosolization is occurring but, epidemiologically, is this really how the virus is spreading?"

Likely not, Hunter agreed. "Aerosol transmission can occur but it is probably a relatively minor route, and it won't make much difference to the course of the pandemic," he said. 

Indeed, several countries have contained the spread of COVID-19 without ever treating the virus as a truly airborne disease. They succeeded in curtailing the outbreak in part through measures that prevent the spread of droplets and exposure to infected surfaces. These methods would have little impact on a virus primarily spread through airborne transmission, Adalja said. (However, Nazaroff countered that social distancing, which was almost universally employed to some degree to limit the spread of SARS-CoV-2,  is not only effective with regards to droplet and close contact transmission, but that it could also reduce the risk of airborne transmission).

Focusing on airborne transmission when it is not a major driver of infection could put undue strain on the healthcare system. Healthcare providers would universally require N95 masks, for instance, which are already in short supply and occasionally unavailable for protecting providers from confirmed airborne diseases such as shingles. "I have had a hard time finding an N95 mask while treating a [shingles] patient during this pandemic," Adalja said.

Part of the confusion may stem from the fact that many of the signatories of the open letter are not infectious disease experts, but experts in fluid mechanics and the study of aerosols. And, while they understand how particles move through the air, their understanding of how those particles fuel disease spread, and the implications of this spread, may prove to have more academic significance than practical value in the midst of a global pandemic, according to Hunter. "Most of them are chemists, engineers, owners of ventilation companies," Hunter said. "They do not have a broad understanding of disease transmission mechanisms … this issue is more nuanced than many of them realize."

Originally published on Live Science.  

Joshua A. Krisch
Live Science Contributor

Joshua A. Krisch is a freelance science writer. He is particularly interested in biology and biomedical sciences, but he has covered technology, environmental issues, space, mathematics, and health policy, and he is interested in anything that could plausibly be defined as science. Joshua studied biology at Yeshiva University, and later completed graduate work in health sciences at Cornell University and science journalism at New York University.