A: It depends what you mean by airborne. Fortunately, the guidance for avoiding the virus in your day-to-day life is the same either way.
On July 4th, a New York Times article reported that 239 scientists from 32 countries had sent an open letter to the World Health Organization (WHO) asserting that “airborne transmission of the coronavirus in indoor settings has been underappreciated” and insisting that WHO guidance be revised accordingly (links to both the NYT articles and the now-published letter below).
This headline caused quite the kerfuffle (h/t Dan Diekema, blog post link below). Haven’t we been told repeatedly that SARS-CoV-2 (the virus that causes COVID-19) primarily spreads through respiratory droplets, not through airborne transmission? Droplet transmission has informed recommendations to wash hands, wear a mask, avoid crowded places, and stay at least 6 feet from people when you do leave your home.
Is all of that wrong? Can the virus actually remain in suspended in air? Do we have to totally rethink our prevention strategies?
No, yes, and not yet.
It turns out that the source of this confusion can be traced back to hospital infection control policies, which treat “droplet transmission” vs “airborne transmission” as dichotomous either/or terms. For simplicity, the bits of stuff that come out of our mouths when we sneeze, cough, sing, speak or breath are called “droplets” when they are big (more than 5 microns); small bits (less than 5 microns) are referred to as “aerosols” (or “droplet nuclei” if you’re feeling fancy). Droplets fall rapidly to the ground once expelled, and therefore only travel about 1 meter. Aerosols can remain suspended in air (sometimes for a long time), and can travel many meters from the origin. In hospitals, the precautions taken against a particular pathogen (like, which kind of mask to use) depend on whether the pathogen is considered droplet transmitted vs. airborne transmitted.
Simple, right? Except it’s not.
In reality, particles aren’t either big droplets or small aerosols. They come in all sizes along a continuum. Medium-sized particles can behave like droplets or like aerosols, and viruses can hitch a ride either way. SARS-CoV-2 is commonly found in respiratory droplets; it can also remain aerosolized (floating in the air) and travel longer distances than we typically imagine droplets traveling. Some medical procedures (intubation, for example) produce aerosolized particles; if a patient undergoing the procedure is shedding virus, those aerosolized particles can and do contain the virus.
So what was the kerfuffle again?
Current WHO guidance about infection control precautions (link below) states that “COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes” and “airborne transmission may be possible in specific circumstances and settings in which procedures or support treatments that generate aerosols are performed.” The guidance goes on to say: “WHO continues to recommend droplet and contact precautions for those people caring for COVID-19 patients. WHO continues to recommend airborne precautions for circumstances and settings in which aerosol-generating procedures and support treatment are performed, according to risk assessment.”
The open letter from the 239 scientists says that this guidance vastly downplays the role of airborne transmission in health care and community settings. While they concede that evidence is not yet conclusive, they do say that there is enough evidence of airborne transmission that WHO should revise their guidance accordingly.
Some infectious disease epidemiologists and other public health researchers pushed back on this, mostly troubled by the emphasis on the term “airborne transmission”. The classic airborne virus is measles — it’s highly infectious (one infected person can infect a whole lot of other people) in part because the virus really can hang out in the air of a room, even hours after the infected person has left. SARS-CoV-2 can be aerosolized, it can travel on smaller particles that travel further and linger longer in the air than droplets, but it is not “airborne” like measles is airborne.
Stay tuned for an update! The WHO has committed to reviewing the evidence on airborne transmission and may update their infection prevention and control precautions recommendations. This could include increased ventilation and air filtration recommendations, and/or universal N95 masking in healthcare settings.
The bottom line: Hanging out in crowded, unventilated rooms with unmasked people who might be shedding virus is still a terrible idea, whether their respiratory gunk is flying straight out of their mouth into your face, or slowly settling down from above for easy inhalation.
Stay SMART: Space, Mask, Air, Restrict social contacts & keep Time with others short!
And yes yes oh my goodness yes: Keep. Washing. Your. Hands.
NYT article
WHO guidance
Open letter from 239 scientists
Dan Diekema blog post
