Is The Coronavirus Airborne Evidence Is Scant Infectious Disease?

August 21, 2020

Experts agree that the new coronavirus behind COVID-19 is spread by direct contact and by large droplets that tend to fall harmlessly to the ground once you sneeze or cough them out. But a recent letter signed by 239 scientists suggests that the virus may also be airborne, lingering for hours in lighter “microdroplets” in the air. The letter challenges the latest guidelines from the World Health Organization (WHO), urging the agency to recognize airborne transmission as an important pathway for COVID-19 infection and to plan accordingly.

William Nazaroff, a contributing author of the letter and professor emeritus of civil and environmental engineering at the University of California, Berkeley, told us that “we need to focus and be aware of all the important transmission pathways in order to make progress” with COVID-19.

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

But epidemiologists and infectious disease experts are cautious about the limited airborne evidence and are concerned that the recent media coverage of the letter will do more harm than good.” It’s a shame that they feel the need to publish it,” Paul Hunter, a professor at the University of East Anglia in the U.K. and a member of the WHO’s Infection Prevention Committee, told Live Science.What’s more, experts say, even if COVID-19 can be transmitted through the air, this is likely to happen only in limited circumstances and in most cases no additional precautions are needed .

“Given the strong evidence for the role of reducing droplet transmission [reducing COVID-19 transmission], throwing something else in there at a critical time will only confuse people and undermine the work of the World Health Organization,” Hunter said.

The strongest evidence for airborne transmission of the novel coronavirus, SARS-CoV-2, is previous work suggesting that the virus in question behind the 2003 SARS outbreak was transmitted through the air. According to the letter, published Monday in the journal Clinical Infectious Diseases, “there is every reason to expect that SARS-CoV-2 behaves similarly, with transmission through airborne microdroplets being an important route.”

Nazarov and colleagues (including Lidia Morawska of the Queensland University of Technology in Australia, the lead author of the open letter) also point to the COVID-19 “super-spread” event that occurred in early March at a Washington choir as evidence of airborne transmission. In another article currently under review, the authors describe a recent outbreak in which 53 of 61 members who attended weekly choir rehearsals were infected with COVID-19, and conclude that airborne microdroplets are the most likely mode of transmission.

“Contortionist thinking is needed to explain this event in any way other than inhalation of shared air,” says Nazaroff.” I’m not an epidemiologist. Suffice it to say that I have not seen a convincing presentation that more than 11 million diagnosed infections can be explained by a combination of droplet, close contact transmission, and fomite [surface] transmission.”

If Nazaroff is correct, then special N95 respirator masks may be needed in addition to the standard surgical masks used by healthcare providers. The social evacuation currently in place may not be enough. Companies may need to retrofit their ventilation systems to eliminate air recirculation and supplement existing ventilation systems with portable air purifiers.

But for epidemiologists, the question is not whether airborne transmission is theoretically possible, or even whether it has occurred in individual cases. The question is whether microdroplets in the air are an important route of infection – important enough to justify the change in WHO guidelines and the major adjustments to sheltering and ventilation protocols.

There, experts say, the evidence is less convincing.

“When infectious disease physicians think about airborne transmission, we’re not talking about an aerobiology experiment. We’re looking for the forces that drive epidemiological transmission”, Dr. Amesh Adalja, an infectious disease specialist at Johns Hopkins University, told Live Science.

While Adalja allows that the novel coronavirus may indeed be transmitted through airborne microdroplets, he does not believe it is an important mode of transmission.” For measles, we know that someone can cough in an elevator and thirty minutes later, even after a brief exposure, that air is still contagious,” he said. That’s not what the experts are seeing with COVID-19.” It’s primarily droplet transmission. You might be able to prove that some aerosolization occurred, but epidemiologically, is that really how the virus spreads?”

Probably not, Hunter agrees.” Aerosol transmission could happen, but it’s probably a relatively small pathway that wouldn’t have much impact on the course of an epidemic,” he said.

In fact, several countries have controlled the spread of COVID-19 without ever treating the virus as a true airborne disease. They have succeeded in containing outbreaks, in part through measures to prevent droplet transmission and contact with infected surfaces. Adalja says these methods have had little impact on a virus that is primarily airborne. (Nazaroff counters, however, that social distancing, almost universally used to limit the spread of SARS-CoV-2, is not only effective for droplet and close contact transmission, but also reduces the risk of airborne transmission).

Focusing on airborne transmission may place unnecessary strain on the healthcare system when airborne transmission is not the primary cause of infection. For example, healthcare providers will have a widespread need for N95 masks, which are already in short supply and occasionally unavailable to protect providers from confirmed airborne diseases such as shingles.” During this pandemic, I’m having a hard time finding N95 masks when treating patients [with shingles],” says Adalja.

Part of the confusion may stem from the fact that many of the signatories to the open letter are not infectious disease experts, but rather experts in fluid mechanics and aerosol research. And while they understand how particles move through the air, according to Hunter, their understanding of how these particles contribute to the spread of disease, and the impact of that spread, may prove to be of more academic than practical value in the context of a global pandemic.” They are mostly chemists, engineers, and owners of ventilation companies,” said Hunter.” They don’t have a broad understanding of the mechanisms of disease transmission…. The problem is more nuanced than many of them realize.”